C4H7 radicals are common intermediates in combustion (for instance, combustion of isobutene), and they also play a role in the formation of aromatic molecules in various astronomical environments. Among the C4H7 isomers, 1-methylallyl (CH2=CH-• CH-CH3, 1-MA) and 2methylallyl (CH2=C(CH3)-• CH2, 2-MA) radicals are methyl-substituted allyl radicals and resonance-stabilized species due to three unpaired electrons in the delocalized π-system. Other C4H7 radicals, such as 2-buten-2-yl (CH3-• C=CH-CH3) and 2-methyl-1-propenyl ( • CH=C(CH3)2), are 1-and 2-methyl-substituted propenyl radicals, respectively, and are intermediates in methylallyl dissociation kinetics. These four isomers of C4H7 serve as an ideal system for studying isomer-dependent photochemistry. Each isomer, representing a local minimum on the global ground-state potential energy surface (PES), provides unique access points to different regions of the excited-state surfaces, possibly leading to distinct dissociation pathways. Our previous studies explored the ultraviolet (UV) photodissociation dynamics of the 1-MA, 1 2-MA, 2 and 2-buten-2yl 3 radicals in the 226-246 nm range. The current study extends this investigation by exploring the UV photodissociation dynamics of the 2-methyl-1-propenyl radical, the fourth isomer in the alkenyl C4H7 series.
The electronic excited states and spectroscopy of both methylallyl radicals have attracted both experimental and theoretical interest. In the UV region of 220-260 nm, 4 2-MA exhibits a primary absorption feature peaking at 238 nm, 5,6 with structures attributed to the Rydberg electronic states, 𝐵 ̃2A1 (3s) (260 nm) [7][8][9][10][11] and 3pz and/or 3px (~240 nm). 12 1-MA has a broad absorption feature peaking at 237.8 nm, 4 attributed to the 2 B1 (3p) Rydberg state, 13 and an absorption due to the à ← 𝑋 ̃ transition at 406-418 nm. 14 Theoretical calculations and spectroscopic studies of 2-buten-2-yl radical were limited, and there were none for the 2-methyl-1-propenyl radical. Koziol et el. 15 conducted equation-of-motion coupled-cluster calculations with single and double excitations (EOM-CCSD), identifying several electronic excited states of 2buten-2-yl with vertical excitation energies in the UV region: π ← n (243 nm), π* ← π (221 nm), 3s ← n (220 nm), 3py ← n (202 nm), 3px ← n (198 nm), and 3pz ← n (194 nm). The H atom photofragment yield (PFY) spectrum of 2-buten-2-yl displayed a broad UV absorption structure peaking at ~234 nm in the photodissociation study of 2-buten-2-yl, possibly due to excitation to the 3py/3px Rydberg states. 3 The potential energy diagram and H-loss dissociation pathways of the relevant C4H7 isomers are shown in Figure 1. The 1-MA and 2-MA radicals are the most stable isomers, while 2-buten-2-yl and 2-methyl-1-propenyl are at higher energies. There are seven different H-loss channels: dimethylvinylidene ((CH3)2C=C∶) + H, methylenecyclopropane (CH2=C(CH2)2) + H, 1methylcyclopropene + H, 1,3-butadiene (CH2=CH-CH=CH2) + H, 1,2-butadiene (CH2=C=CH-CH3) + H, 1-butyne (CH≡C-CH2-CH3) + H, and 2-butyne (CH3-C≡C-CH3) + H. Additionally, there are two CH3-elimination pathways: CH3 + propyne (CH3-C≡CH) and CH3 + allene (CH2=C=CH2). The predominant H-loss channel in photodissociation of 2-MA via the 3s Rydberg state was H + methylenecyclopropane, which occurred on the ground state following internal conversion. 9 The UV photodissociation of 2-MA (at 226-244 nm) via the 3p Rydberg state showed bimodal H-atom translational energy and angular distributions. 2 Slow, isotropic H-atoms (β ≈ 0, branching ratio (BR) ~98%) resulted from internal conversion followed by ground-state dissociation to H + methylenecyclopropane. Fast, slightly anisotropic H-atoms (β ≈ -0.2, BR ~2%) likely arose from direct H-loss after isomerization to 1-MA on the excited states, forming 1,3butadiene + H. For 1-MA, the à ← 𝑋 ̃ excitation (406-418 nm) primarily yielded 1,3-butadiene + H. 14 In the 1-MA photodissociation at 226-244 nm via the 3p states, slow isotropic H-loss (BR ~93%) indicated statistical ground-state unimolecular dissociation to 1,3-butadiene + H after internal conversion, while fast anisotropic H-atoms (β ≈ -0.23, BR ~7%) suggested non-statistical dissociation via the excited state or a conical intersection, also yielding 1,3-butadiene + H. 1 Compared to 1-MA and 2-MA, the dissociation dynamics of 2-buten-2-yl were less explored. McCunn et al. studied the secondary dissociation pathways of 2-buten-2-yl, generated through 193 nm photodissociation of 2-chloro-2-butene. 16 Upon losing a Cl atom, internally hot 2-buten-2-yl underwent three possible dissociation routes: CH3 elimination, leading to the formation of propyne + CH3; H-atom loss, producing 1,2-butadiene + H; and H-atom loss, yielding 2-butyne + H. CH3 elimination was observed to be the most favorable pathway, significantly surpassing the H-atom loss channels. However, Rice-Ramsperger-Kassel-Marcus (RRKM) calculations indicated that higher internal energy shifted the balance toward the H-atom loss pathways. 16 Similar to 1-MA and 2-MA, UV photodissociation of 2-buten-2-yl at 226-246 nm exhibited bimodal H-atom translational energy distributions. 3 The primary pathway (ET peak ~ 7 kcal/mol, fraction of average ET in the total excess energy, fT ~ 0. 12) had an isotropic angular distribution (β ~ 0), consistent with ground-state dissociation of hot 2-buten-2-yl radical following internal conversion, producing 2-butyne + H (~84%) and 1,2-butadiene + H (~16%). A secondary fast pathway (ET peak ~35 kcal/mol, fT ~ 0.40-0.46) with an isotropic angular distribution suggested direct dissociation on a repulsive surface, yielding 1,2-butadiene + H. 3 To date, no experimental studies have been reported on the dissociation of 2-methyl-1propenyl. The 2-methyl-1-propenyl radical could directly lose a CH3 radical to form propyne (CH3-C≡CH) + CH3 (this serves as a pathway for the CH3 loss of 2-MA following its isomerization to 2-methyl-1-propenyl). The 2-methyl-1-propenyl radical could also directly lose an H atom to form dimethylvinylidene ((CH3)2C=C∶) + H, but this is a high energy pathway (dissociation energy is calculated to be 83.0 kcal/mol in this work, see Methods for details). As shown in Figure 1, from the ground electronic state, 2-methyl-1-propenyl can isomerize to 2-MA over a barrier of 38.4 kcal/mol and then proceed with the same H-loss dissociation channels of 2-MA. 9,[17][18][19][20][21][22]23 2-MA cannot directly lose an H atom, and the lowest H-loss pathway for 2-MA involves cyclization to 1-methylcyclopropyl, followed by loss of a methyl H atom to produce methylenecyclopropane + H, with a total energy barrier of 67.2 kcal/mol relative to 2-MA or 44.2 kcal/mol to 2-methyl-1propenyl. Alternatively, 1-methylcyclopropyl can lose an H atom on the ring to yield H + 1methylcyclopropene, with an overall energy barrier of 54.8 kcal/mol to 2-methyl-1-propenyl. By undergoing a 1,2-methyl shift, 2-MA can isomerize to 1-buten-2-yl (CH2= • C-CH2-CH3), overcoming a high barrier of 79.3 kcal/mol relative to 2-MA. 1-buten-2-yl can then lose an H atom to form H + 1,2-butadiene or H + 1-butyne. Further, 1-buten-2-yl can isomerize to 1-MA, which may lose a methyl H atom to produce H + 1,3-butadiene or lose an H atom on the allyl frame to generate H + 1,2-butadiene. The H-atom product channels and energetics for 2-methyl-1-propenyl from Figure 1 are summarized below. 2-methyl-1-propenyl → methylenecyclopropane + H ΔHrxn,0 = 40.8 kcal/mol → 1-butyne + H ΔHrxn,0 = 34.5 kcal/mol → 1,2-butadiene + H ΔHrxn,0 = 33.3 kcal/mol → 2-butyne + H ΔHrxn,0 = 29.4 kcal/mol → 1,3-butadiene + H ΔHrxn,0 = 21.3 kcal/mol → 1-methylcyclopropene + H ΔHrxn,0 = 51.7 kcal/mol → dimethylvinylidene + H ΔHrxn,0 = 83.0 kcal/mol Compared to the above H-loss channels, the direct CH3-loss pathway (producing CH3 and propyne) is more favorable for 2-methyl-1-propenyl, with a barrier of 33.6 kcal/mol (~ 5 kcal/mol lower than isomerization to 2-MA). 19 This pathway would compete with the most energetically favored H-atom product channel (H + methylenecyclopropane), which has an overall barrier pf 44.2 kcal/mol. In this work, the UV photodissociation dynamics of jet-cooled 2-methyl-1-propenyl radicals were investigated in the 226-248 nm region using the high-n Rydberg time-of-flight (HRTOF) technique. The H-atom PFY spectra revealed the UV absorption features of 2-methyl-1-propenyl for the first time. From the H-atom product TOF spectra, center-of-mass translational energy distributions and angular distributions of the H + C4H6 products were obtained. This study represents the first detailed investigation of the photodissociation dynamics of 2-methyl-1propenyl radicals upon UV excitation.
The HRTOF technique and experimental setup have been described in previous publications. [24][25][26][27] The 2-methyl-1-propenyl radicals were generated using 1-chloro-2methylpropene (98%, Sigma-Aldrich) or 1-bromo-2-methylpropene (98%, Sigma-Aldrich) as precursors. A pulsed 2-methyl-1-propenyl radical beam was produced by photolyzing a ~2%
precursor mixture in He (total pressure of ~120 kPa) with 193 nm radiation from an ArF excimer laser. Following generation, the 2-methyl-1-propenyl radicals were cooled via supersonic expansion in the molecular beam and collimated by a 1 mm skimmer 2.8 cm downstream into the reaction chamber. The radical beam was characterized using 121.6 nm vacuum ultraviolet (VUV) photoionization time-of-flight mass spectrometry (TOFMS). In the reaction region, the radicals were photodissociated by slightly focused UV laser radiation (at 226-248 nm, 0.25-1.5 mJ/pulse, linewidth 0.3 cm -1 ). A Fresnel-Rhomb achromatic λ/2 plate was used to rotate the polarization of the UV photodissociation laser for H-atom product angular distribution measurements. The H atoms produced from the photodissociation of 2-methyl-1-propenyl were tagged via two-color resonant excitation, first from the 1 2 S to 2 2 P state using the H-atom Lyman-α transition (121.6 nm)
and then to a high-n Rydberg state with UV radiation at 366.3 nm. A fraction of these metastable Rydberg H atoms drifted with their nascent velocities to a microchannel plate (MCP) detector positioned perpendicular to the molecular beam. The H atoms were field-ionized near the detector and recorded. The nominal flight length of 37.2 cm was calibrated using the 236-nm photodissociation of HBr, utilizing its well-known dissociation and spin-orbit splitting energies of the Br( 2 P3/2) and Br( 2 P1/2) products. The ion signals were amplified using a fast preamplifier, and the H-atom TOF spectra were recorded and averaged using a multichannel scaler. The TOF spectra were averaged over 110 5 to 510 5 laser shots each.
To support and align with our experimental observations, the electronic transitions from the ground electronic state to the excited states of 2-methyl-1-propenyl were analyzed using multiple quantum chemistry approaches. Ground state structures of the 2-methyl-1-propenyl radical were optimized with CCSD/aug-cc-pVDZ. Vertical excitation energies from the ground state to the six lowest excited electronic states were calculated with the time-dependent densityfunctional theory (TDDFT) (CAM-B3LYP) and EOM-CCSD, employing various basis sets, including d-aug-cc-pVXZ (X=D, T, Q). 28 To provide a clear and qualitative understanding of the electronic excitations, natural transition orbitals (NTOs) 29 were analyzed using CAM-B3LYP/6-31++G. All ab initio calculations were performed with Gaussian 16, and the results, including optimized geometries, excitation energies, transition dipole moments, and NTO analyses, are provided in Electronic Supplementary Information (ESI) (Table S1, S4, and S5 and Figure S1 and S2). As the energy of dimethylvinylidene was not available in literature, the dissociation energy of 2-methyl-1-propenyl to (CH3)2C=C∶ + H was evaluated using CCSD(T)/cc-pVDZ single-point energies computed at the CAM-B3LYP/6-311++G(d,p)-optimized geometries (Table S2 and S3).
To confirm the production of the 2-methyl-1-propenyl radical, the difference VUV photoionization mass spectrum of the 1-chloro-2-methylpropene precursor is presented in Figure peak at 55 m/z, corresponding to the formation of the 2-methyl-1-propenyl radical, and two negative peaks at 90 and 92 m/z, indicating depletion of the parent 1-chloro-2-methylpropene precursor.
The TOF spectra of the H-atom products from the photodissociation of 2-methyl-1propenyl were measured in the wavelength region of 226-248 nm, with the photodissociation laser polarization set parallel and perpendicular to the flight path. Background TOF spectra were carefully analyzed and subtracted to isolate the correct H-atom signal associated with the 2-methyl-1-propenyl radical photodissociation, as outlined in previous studies. 24,27,30,31 The primary background signals originated from the H-atom products of the 2-methyl-1-propenyl precursor photodissociation. Figure 3 illustrates the H-atom TOF spectra following 236 nm photodissociation of two precursors: 1-chloro-2-methylpropene and 1-bromo-2-methylpropene.
Both spectra exhibited a broad feature peaking at approximately 37 μs, confirming the formation of H atoms from the 2-methyl-1-propenyl radical photodissociation. In Figure 3 H-atom signals were linearly dependent on the photolysis laser power. To minimize contributions from multiphoton processes, low photolysis energies (< 0.85 mJ/pulse) were employed throughout this study. The H-atom TOF spectra recorded in the UV photolysis wavelength range of 226-248 nm were integrated, normalized for photolysis power, and scaled to the 236 nm data, which served as a reference to monitor potential experimental drift. This process produced the H-atom PFY spectrum for the 2-methyl-1-propenyl radical (Figure 4), characterized by a broad absorption feature peaking near 240 nm and exhibiting an increased intensity below 228 nm. To analyze the photodissociation behavior and electronic transitions of the radical observed in the PFY spectrum, the excited electronic states of 2-methyl-1-propenyl were calculated using two quantum chemical methods: TDDFT and EOM-CCSD. Comprehensive calculation results, including the ground-state structure of 2-methyl-1-propenyl, excited-state energies, and transition dipole moments, are provided in the ESI (Table S1, S4, and S5 and Figure S1 and S2). The calculated excitation energies for the six excited electronic states (D1-D6) of 2-methyl-1-propenyl suggest that the UV excitation observed in the 226-248 nm region with the increased intensity below 228 nm could correspond to the edge of the strong absorption peak of the fourth excited state (D4). This is primarily attributed to its significantly larger oscillator strength, in combination with the electronic excitation energy aligning with the observed increase in UV absorption below 228 nm (Figure 4).
Various methods and basis sets were employed in the above predictions, including CAM-B3LYP/d-aug-cc-pVQZ, CAM-B3LYP/aug-cc-pVQZ, and EOM-CCSD/d-aug-cc-pVDZ.
Notably, the EOM-CCSD/d-aug-cc-pVTZ method estimates the vertical excitation energy of the D4 excited state to be 201.4 nm. The d-aug-cc-pVXZ basis set, which incorporates two shells of each angular momentum, offers improved accuracy for Rydberg excited states compared to the aug-cc-pVXZ basis set, which includes only one shell of each angular momentum. Analysis on NTOs reveals that the D4 state has a dominant 3s Rydberg character (the n → 3s transition, Table S5), and the involved Rydberg orbital appears as a diffuse, roughly spherical cloud centered around the radical site. The computed transition dipole moment from the ground state to D4 lies within the propenyl plane (Tables S5; Figure S1). Alternatively, the board absorption feature centered around 240 nm could also be due to the third excited state (D3), via the n → π * transition. This transition has an oscillator strength ~ 15 times smaller than that of the n → 3s transition, but its calculated vertical transitions (Table S5) are in a very good agreement with the ~ 240-nm peak position of the observed broad feature. The calculated transition dipole moment of the n → π * transition (to D3) lies perpendicular to the propenyl plane (Table S5; Figure S2).
To determine the energy distribution of the photodissociation products, the net H-atom TOF spectra from jet-cooled 2-methyl-1-propenyl photodissociation are transformed to the centerof-mass (CM) product translational energy distributions, P(ET)'s. The CM translational energy, ET, is derived from the H-atom flight time, tH, using the following equation:
where EH is the laboratory frame translational energies of the H-atom photofragment and L is the length of the TOF path. The P(ET) distributions for the photodissociation of 2-methyl-1-propenyl at 236 nm are shown in Figure 5 (a) and (b). They feature a broad peak located at a low translational energy of around 7 kcal/mol and extend up to ~80 kcal/mol, which is close to the maximum available energy for the methylenecyclopropane + H product channel (80.35 kcal/mol). The maximum available energies for other H-atom product channels at 236 nm photodissociation are also indicated in Figure 5: 69.45 kcal/mol for 1-methylcyclopropene + H, 86.65 kcal/mol for 1butyne + H, 87.85 kcal/mol for 1,2-butadiene + H, 91.75 kcal/mol for 2-butyne + H, and 99.85 kcal/mol for 1,3-butadiene + H, although there is little or no intensity in the P(ET) distributions for the higher ET product channels (1-butyne + H, 1,2-butadiene + H, 2-butyne + H, and 1,3-butadiene + H). The maximum available energy was calculated as ET,max = hν -D0, by subtracting the
where β is the anisotropy parameter (-1 ≤ β ≤ 2), θ represents the angle between the electric vector of the polarized laser radiation (E) and the H-atom's recoiling velocity vector (aligned with the TOF axis), and P2(cosθ) is the second Legendre polynomial. Using this expression, the anisotropy parameter 𝛽(𝐸 T ) was determined from the P(ET) distributions in Figure 5 propenyl dissociation and direct HCl dissociation. Since direct HCl dissociation typically produces a negative β, 33,34 this is consistent with the sharp peaks observed between 31-36 kcal/mol and 38-44 kcal/mol in β in Figure 5 (c). To address these contributions, four Gaussian functions were applied to fit and separate the peaks in the perpendicular spectrum (inset in Figure 5 (b)). After subtracting these fitted components, the corrected β (red line in Figure 5 (c)) remains near zero across the 30-45 kcal/mol range. This analysis establishes that the dissociation of 2-methyl-1propenyl results in an isotropic angular distribution (β ≈ 0) across the entire energy range, with deviations above 50 kcal/mol attributed to a reduced signal-to-noise ratio.
The P(ET) distributions indicate a moderate release of product kinetic energy. At 236 nm, the average translational energy release, ⟨ET⟩, is 10.7 kcal/mol, with the fraction of the average translational energy release in the total available energy, ⟨fT⟩, calculated as 0.13, assuming the lowest energy barrier pathway that leads to methylenecyclopropane + H. The fT for the photolysis region of 226-248 nm remains modest, increasing slightly from 0.13 to 0.15 as illustrated in Figure 6. The small ⟨fT⟩ values suggest that the C4H6 product is highly internally excited, aligning with a hot radical unimolecular dissociation mechanism.
The H-atom yield time profile during the UV photolysis of 2-methyl-1-propenyl was examined, which was recorded by varying the delay between the photolysis and probe lasers.
Figure 7 illustrates this time profile at the photolysis wavelength of 236 nm, constructed by integrating the HRTOF spectra as a function of the delay time. The initial increase in the signal reflected the rate at which H-atoms were produced from 2-methyl-1-propenyl, while the subsequent signal decay was attributed to H-atoms leaving the laser interaction region. To estimate the unimolecular dissociation rate of the 2-methyl-1-propenyl radical, the time profile was fitted using Equation ( 3), which is based on previous work from Chen's group. 35
In this equation, kH denotes the dissociation rate constant for H-atom production, and the constants a and b characterize the plateau width and the decay of the signal. The solid line in Figure 7 represents the fitted model, yielding a dissociation rate constant of kH ~ 6.6 × 10 7 s -1 at 236 nm.
Due to the 10-ns time resolution limitation of the pump and probe lasers, this value serves as a lower bound for the actual dissociation rate constant.
The UV photodissociation of jet-cooled 2-methyl-1-propenyl radicals was studied for the first time in the range of 226-248 nm. The C4H6 + H photodissociation channels were directly observed in the H-atom TOF spectra. The H-atom spectra obtained at 236 nm from both 1-chloro-2-methylpropene and 1-bromo-2-methylpropene precursors (Figure 3) exhibit similar profiles, confirming that the H-atom signals arise from the same species of 2-methyl-1-propenyl. The Hatom PFY spectrum of 2-methyl-1-propenyl, recorded in the UV photolysis range of 226-248 nm, exhibits a broad absorption feature centered at ~240 nm, with increasing intensity toward shorter wavelengths (Figure 4). This represents the first reported absorption characteristic for the 2methyl-1-propenyl radical. Given that the H-loss channels are less favorable than the CH3 loss pathway, 19 the observed H-atom PFY spectrum provides only a component of the UV absorption of 2-methyl-1-propenyl. Our electronic state calculations point to the D4 (3s Rydberg) excited state as the most likely candidate responsible for the absorption in the 226-248 nm range, as it has a much larger oscillator strength compared to other excited states and its excitation energy matching the observed increase in absorption at shorter wavelengths, and the D3 (π*) excited state possibly also contributing to the broad absorption peaking around 240 nm. The broad feature indicates a rapid decay of the excited state; as no direct dissociation (with fast, repulsive energy release) was observed, the rapid decay of the excited state suggests that H-atom photodissociation likely occurs after the electronically excited 2-methyl-1-propenyl undergoes internal conversion to the ground electronic state, followed by unimolecular decomposition on the ground-state surface.
The TOF spectra of 2-methyl-1-propenyl in the region of 226-248 nm are similar, only containing a broad feature peaking at ~37 μs. The corresponding P(ET) distributions show a broad feature peaking at a low energy of ~7 kcal/mol (Figure 5), with a modest fT of 0.13-0.15 (Figure 6). This low-energy broad feature indicates that the photodissociation of 2-methyl-1-propenyl follows a statistical energy distribution, consistent with the dissociation of a hot radical after internal conversion. With both the P(ET)'s and fT's supporting a statistical dissociation mechanism, the H-atom product is expected to be mainly from the lowest energy barrier pathway on the ground state PES. As the direct H-atom loss of 2-methyl-1-propenyl on the ground electronic state to form dimethylvinylidene requires much higher energy (83 kcal/mol), 2-methyl-1-propenyl must first isomerize to 2-MA and then proceed via other H-atom loss pathways (Figure 1). The most probable H-atom loss channel is the formation of methylenecyclopropane + H after 2-methyl-1-propenyl isomerization to 2-MA and then to 1-methylcyclopropyl, as this pathway has the lowest overall energy barrier for H elimination (44.2 kcal/mol relative to 2-methyl-2-propenyl) that is 12.3 kcal/mol lower than that of the other isomerization of 2-MA to 1-buten-2-yl. The 1buten-2-yl radical could further isomerize to 1-MA and then dissociate to the lowest-energy Hatom product channel (1,3-butadiene + H) on the C4H7 PES. Although the energy available from the UV excitation is sufficient for 2-methyl-1-propenyl to overcome the higher isomerization barrier to 1-buten-2-yl and then to multiple dissociation channels, in the statistical unimolecular dissociation of the hot radical, these other H-atom product channels (1,3-butadiene + H, etc.) would contribute much less to the observed signals than the methylenecyclopropane + H product channel that has the lowest overall energy barrier. This is consistent with the P(ET) distributions that extend up to ~80 kcal/mol, close to the maximum available energy for the methylenecyclopropane + H product channel instead of those of other H-atom product channels such as 1,2-butadiene + H, 1-butyne + H, and 1,3-butadiene + H (Figure 5). The peak of ~7 kcal/mol in the P(ET) distributions is also consistent with the methylenecyclopropane + H product channel that has a 3.4 kcal/mol exit channel barrier.
The angular distribution for 2-methyl-1-propenyl at 236 nm in Figure 6 shows an isotropic distribution with β ~ 0. The isotropic distribution implies a photodissociation timescale that is longer than the rotational period of the radical (~ ps). This ps photodissociation timescale is also consistent with a statistical photodissociation mechanism where H-atom loss occurs on the ground electronic state after internal conversion from the excited electronic state. The pump-probe delay experiment in Figure 7 was used to evaluate the rate constant for H-atom appearance and provides a lower limit of 6.6 10 7 s -1 and is consistent with the timescale suggested by the isotropic angular distribution.
Finally, it is insightful to compare the results from UV photodissociation studies of the four C4H7 isomers, three of which (1-MA, 2-MA, and 2-buten-2-yl) have been previously investigated also in the 226-246 nm range. [1][2][3] The P(ET) distributions of all four isomers exhibit a dominant broad feature peaking at low energies (~7 kcal/mol) with an isotropic angular distribution, characteristic of hot radical unimolecular dissociation after internal conversion from the electronic excited states. The fT for the isotropic, slow component ranges from 0.16-0.19 for 2-MA, 2 0.13-0.17 for 1-MA, 1 0.11-0.12 for 2-buten-2-yl, 3 and 0.13-0.15 for 2-methyl-1-propenyl (this work).
These similarly low fT values also support that photodissociation occurs following internal conversion from the excited electronic state to a highly vibrationally excited ground electronic state, leading to the statistically favored H-atom product channel. These unimolecular dissociation dynamics observed in the methyl-substituted allyl and propenyl radicals are consistent with those previously reported for the allyl and 1-propenyl radicals. 31,36 Unlike the other C4H7 isomers, which exhibit a small fast and repulsive dissociation pathway to the lowest energy product channels such as 1,2-butadiene + H, 1-butyne + H, and 1,3-butadiene + H, 2-methyl-1-propenyl does not show a discernable fast and repulsive H-loss component to these lowest energy product channels. A potential direct dissociation pathway involves H-atom loss from the C=C bond, forming the carbene species (CH3)2C=C∶ (dimethylvinylidene) (from the excited state 2-methyl-1-propenyl).
Although the dissociation energy required for this process (~83 kcal/mol) is accessible in the studied wavelength region (116.2-126.5 kcal/mol excitation energy), comparable to the ~78 kcal/mol needed for vinyl (CH2=CH • ) to directly dissociate into vinylidene (CH2=C∶), 23,37 the contribution of the direct repulsive dissociation pathway from the excited state 2-methyl-1propenyl is expected to be negligible in the translational energy ET range of 30-45 kcal/mol. This is speculated by the analogous fact that the branching ratio of CH2=C∶ for vinyl dissociation at 243 nm is only ~1%. 38 Furthermore, the absorption band of 2-methyl-1-propenyl observed at 226-248 nm is likely associated with the D4 (3s) Rydberg state or D3 (π*) state. The n → 3s transition has its dipole moment oriented within the propenyl (the (C)2C=C) plane. As a result, rapid H-loss from the C=C bond in 2-methyl-1-propenyl is predicted to yield a slightly negative anisotropy parameter (β = -0.11). Similarly, the n → π* transition has the dipole moment perpendicular to the propenyl plane, and the rapid H-loss from the C=C bond in 2-methyl-1-propenyl would yield a significantly negative anisotropy parameter (β -1). The combination of an expected minimal contribution from this pathway, the essentially isotropic angular distribution of the H-atom photofragments, and absence of any discernable extra feature in the 30-48 kcal/mol region of the P(ET) distributions renders the direct repulsive carbene + H product channel to be negligible or nonexistent in the studied wavelength range.
The UV photodissociation dynamics of the four C4H7 isomers exhibit isomer-specific behavior due to their distinct geometries and interactions with the ground-state PES. Variations in dissociation pathways and isomerization barriers lead to differences in both slow and fast H-atom elimination processes. For instance, 2-methyl-1-propenyl must first isomerize to 2-MA and then to 1-buten-2-yl and 1-MA, in order to produce high ET H-loss channels H + 1,3-butadiene, H + 2butyne, and H + 1,2-butadiene repulsively on the excited states; this multistep process would result a slower dissociation and the absence of a significant fast H-loss channels. In contrast, the fast Hloss pathway is observed in 1-MA, 2-MA, and 2-buten-2-yl, all associated with 3p Rydberg states, but with distinct translational energy releases and angular distributions: fT ~ 0.62-0.72 and β ~ -0.23 for 1-MA, 1 fT ~ 0.58-0.68 and β ~ -0.2 for 2-MA, 2 and fT ~ 0.40-0.46 and β ~ 0 for 2buten-2-yl. 3 The lack of a fast channel in 2-methyl-1-propenyl, linked to its 3s Rydberg state and/or π* excited state and its different structure that is not well connected to the H + 1,3-butadiene, H + 2-butyne, and H + 1,2-butadiene product channels, highlights the role of structural and energetic constraints. These findings demonstrate how isomer-specific geometries and energy landscapes govern dissociation behavior, with the Rydberg state dynamics playing a key role in shaping the observed pathways.
The H-atom product channels of the 2-methyl-1-propenyl radical were investigated at UV wavelengths of 226-248 nm, marking the first reported study of its UV photodissociation dynamics. The H-atom PFY spectra reveal a broad UV absorption feature around 240 nm with increasing intensity at shorter wavelengths of < 228 nm. Comparison with theoretical calculations suggests that the photodissociation of 2-methyl-1-propenyl in this wavelength range is initiated from the 3s Rydberg state or also possibly from the π* state. The H atom product P(ET) distributions exhibit a modest kinetic energy release, peaking ~ 7 kcal/mol, with fT remaining nearly constant at ∼0.13-0.15 in the photolysis wavelength range 226-248 nm. The isotropic angular distribution of the H-atom photofragments indicates that the overall photodissociation process occurs on a timescale exceeding a rotational period of the parent radical. The proposed mechanism involves hot radical dissociation on the ground electronic state following internal conversion from the electronically excited states. These findings are consistent with the dissociation of a highly vibrationally excited 2-methyl-2-propenyl radical in the ground electronic state, which likely isomerizes to 2-MA before producing methylenecyclopropane + H.
10 20 30 40 50 60 70 80 110 2-methyl-1-propenyl
1-methylallyl 60.7 23.3 61.7 60.5 47.0 60.4 64.8 79.8 67.5 78.1 49.9 57.8 64.1 75.0 27.0 22.9 44.6 + H + H + H Energy (kcal/mol) + H 0.5 0.0 + H 56.6 2-methylallyl 20.6 52.7 61.5 56.5 58.9 + H 2-buten-2-yl 64.2 50.1 78.4 + H 106.3* 30.0 Figure 1. Potential energy diagram of C4H7 dissociation pathways. The energetics and pathways are based on the theoretical calculations in Reference 19. *The dissociation energy of 2-methyl-1propenyl to singlet dimethylvinylidene (CH3)2C=C∶ + H here is obtained from CCSD(T)/cc-pVDZ single point calculations on geometries optimized at the CAM-B3LYP/6-311++G(d,p) level. 40 50 60 70 80 90 100 110 Intensity (arb. units) Mass (amu) molecular beam using 1-chloro-2-methylpropene precursor in the He carrier gas. This is the difference spectrum with 193 nm radical producing photolysis laser on minus off. The main product is 2-methyl-1-propenyl at m/z = 55 amu, while the depleted 1-chloro-2-methylpropene parent peaks are at m/z = 90 and 92 amu (the positive peak in between is due to imperfect subtraction of the large m/z 90 and 92 amu parent peaks between radical producing photolysis laser on and off). radical at 236 nm, produced from 193 nm photolysis of 1-chloro-2-methylpropene (a) and 1bromo-2-methylpropene (b) precursors. The polarization E vector of the 236 nm radiation is set to be perpendicular (θ = 90°) and parallel (θ = 0°) to the TOF axis. These are the net H-atom TOF with the 193 nm photolysis radiation on minus off. The sharp peaks are caused by the photodissociation of HCl in (a) and HBr in (b), the byproduct from photodissociation of 1-chloro-2-methylpropene and 1-bromo-2-methylpropene precursor, respectively. 28 225 230 235 240 245 250 0.0 0.4 0.8 1.2 1.6 Relative Intensity (arb. units) Wavelength (nm) Figure 4. H-atom product yield (PFY) spectrum as a function of photolysis excitation energy in the region of 226-248 nm. The open circles (ο) represent the integrated HRTOF signals using the 1-chloro-2-methylpropene precursor. 29 30 40 50 30 40 50 0 20 40 60 80 100 -1.0 -0.5 0.0 0.5 1.0 50 +H +H +H +H P(E T ) (a) Parallel (q = 0°) +H 4 +H 50 CM Translational Energy (kcal/mol) +H +H +H +H (b) Perpendicular (q = 90°) +H 4 +H b (c) Anisotropy parameter b with HCl b without HCl Figure 5. Center-of-mass product translational energy distribution, P(ET), of 2-methyl-1-propenyl radical at 236 nm are shown for (a) parallel and (b) perpendicular polarizations relative to the TOF axis. Vertical lines mark the maximum translational energies for the H-atom product channels. The sharp peaks observed between 30-45 kcal/mol arise from the photodissociation of HCl (v" = 2-3), resulting from H-atom abstraction from the parent molecule, 1-chloro-2-methylpropene. Additionally, the anisotropy parameter in (c) is derived from the data in (a) and (b). Further details are provided in the text. 30 225 230 235 240 245 250 0.00 0.05 0.10 0.15 0.20 0.25 Average f T Photolysis Wavelength (nm) Figure 6. Photolysis wavelength and fraction of average translational energy release in the total available energy, fT, in the UV photodissociation of the 2-methyl-1-propenyl radical. The average translational energies are calculated from the experimental P(ET) distributions. The total available energy at each photolysis wavelength is derived from the corresponding photon energy and the dissociation energy of 2-methyl-2-propenyl to methylenecyclopropane + H. The error bars indicate statistical uncertainty (1σ) from multiple measurements (the varying size of the error bars is due