An official website of the United States government
Context.Recent JWST observations have measured the ice chemical composition towards two highly extinguished background stars, NIR38 and J110621, in the Chamaeleon I molecular cloud. The observed excess of extinction on the long-wavelength side of the H2O ice band at 3 μm has been attributed to a mixture of CH3OH with ammonia hydrates NH3·H2O), which suggests that CH3OH ice in this cloud could have formed in a water-rich environment with little CO depletion. Laboratory experiments and quantum chemical calculations suggest that CH3OH could form via the grain surface reactions CH3+ OH and/or C + H2O in water-rich ices. However, no dedicated chemical modelling has been carried out thus far to test their efficiency. In addition, it remains unexplored how the efficiencies of the proposed mechanisms depend on the astrochemical code employed. Aims.We modelled the ice chemistry in the Chamaeleon I cloud to establish the dominant formation processes of CH3OH, CO, CO2, and of the hydrides CH4and NH3(in addition to H2O). By using a set of state-of-the-art astrochemical codes (MAGICKAL, MONACO, Nautilus, UCLCHEM, and KMC simulations), we can test the effects of the different code architectures (rate equation vs. stochastic codes) and of the assumed ice chemistry (diffusive vs. non-diffusive). Methods.We consider a grid of models with different gas densities, dust temperatures, visual extinctions, and cloud-collapse length scales. In addition to the successive hydrogenation of CO, the codes’ chemical networks have been augmented to include the alternative processes for CH3OH ice formation in water-rich environments (i.e. the reactions CH3+ OH → CH3OH and C + H2O → H2CO). Results.Our models show that the JWST ice observations are better reproduced for gas densities ≥105cm−3and collapse timescales ≥105yr. CH3OH ice formation occurs predominantly (>99%) via CO hydrogenation. The contribution of reactions CH3+ OH and C + H2O is negligible. The CO2ice may form either via CO + OH or CO + O depending on the code. However, KMC simulations reveal that both mechanisms are efficient despite the low rate of the CO + O surface reaction. CH4is largely underproduced for all codes except for UCLCHEM, for which a higher amount of atomic C is available during the translucent cloud phase of the models. Large differences in the predicted abundances are found at very low dust temperatures (Tdust<12 K) between diffusive and non-diffusive chemistry codes. This is due to the fact that non-diffusive chemistry takes over diffusive chemistry at such low Tdust. This could explain the rather constant ice chemical composition found in Chamaeleon I and other dense cores despite the different visual extinctions probed.
more »
« less
A multi‐descriptor analysis of substituent effects on the structure and aromaticity of benzene derivatives: π‐Conjugation versus charge effects
Abstract This work provides a detailed multi‐component analysis of aromaticity in monosubstituted (X = CH3, C, C, NH2, NH−, NH+, OH, O−, and O+) andpara‐homodisubstituted (X = CH3, CH2, NH2, NH, OH, and O) benzene derivatives. We investigate the effects of substituents using single‐reference (B3LYP/DFT) and multireference (CASSCF/MRCI) methods, focusing on structural (HOMA), vibrational (AI(vib)), topological (ELFπ), electronic (MCI), magnetic (NICS), and stability (S0–T1splitting) properties. The findings reveal that appropriateπ‐electron‐donating andπ‐electron‐accepting substituents with suitable size and symmetry can interact with theπ‐system of the ring, significantly influencingπ‐electron delocalization. While the charge factor has a minimal impact onπ‐electron delocalization, the presence of apzorbital capable of interacting with theπ‐electron delocalization is the primary factor leading to a deviation from the typical aromaticity characteristics observed in benzene.
more »
« less
- Award ID(s):
- 2107923
- PAR ID:
- 10483821
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Journal of Computational Chemistry
- ISSN:
- 0192-8651
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract The type II polyproline helix (PPII) is a fundamental secondary structure of proteins, important in globular proteins, in intrinsically disordered proteins, and at protein‐protein interfaces. PPII is stabilized in part byn→π* interactions between consecutive carbonyls, via electron delocalization between an electron‐donor carbonyl lone pair (n) and an electron‐acceptor carbonyl (π*) on the subsequent residue. We previously demonstrated that changes to the electronic properties of the acyl donor can predictably modulate the strength ofn→π* interactions, with data from model compounds, in solution in chloroform, in the solid state, and computationally. Herein, we examined whether the electronic properties of acyl capping groups could modulate the stability of PPII in peptides in water. InX−PPGY‐NH2peptides (X=10 acyl capping groups), the effect of acyl group identity on PPII was quantified by circular dichroism and NMR spectroscopy. Electron‐rich acyl groups promoted PPII relative to the standard acetyl (Ac−) group, with the pivaloyl andiso‐butyryl groups most significantly increasing PPII. In contrast, acyl derivatives with electron‐withdrawing substituents and the formyl group relatively disfavored PPII. Similar results, though lesser in magnitude, were also observed inX−APPGY‐NH2peptides, indicating that the capping group can impact PPII conformation at both proline and non‐proline residues. The pivaloyl group was particularly favorable in promoting PPII. The effects of acyl capping groups were further analyzed inX–DfpPGY‐NH2andX−ADfpPGY‐NH2peptides, Dfp=4,4‐difluoroproline. Data on these peptides indicated that acyl groups induced order Piv‐ > Ac‐ > For‐. These results suggest that greater consideration should be given to the identity of acyl capping groups in inducing structure in peptides.more » « less
-
Abstract The effects on the C−I⋅⋅N halogen bond between iodobenzene and NH3of placing various substituents on the phenyl ring are monitored by quantum calculations. Substituents R=N(CH3)2, NH2, CH3, OCH3, COCH3, Cl, F, COH, CN, and NO2were each placed ortho, meta, and para to the I. The depth of the σ‐hole on I is deepened as R becomes more electron‐withdrawing which is reflected in a strengthening of the halogen bond, which varied between 3.3 and 5.5 kcal mol−1. In most cases, the ortho placement yields the largest perturbation, followed by meta and then para, but this trend is not universal. Parallel to these substituent effects is a progressive lengthening of the covalent C−I bond. Formation of the halogen bond reduces the NMR chemical shielding of all three nuclei directly involved in the C−I⋅⋅N interaction. The deshielding of the electron donor N is most closely correlated with the strength of the bond, as is the coupling constant between I and N, so both have potential use as spectroscopic measures of halogen bond strength.more » « less
-
The ability of the CH group to act as proton donor is now widely accepted, even if the H bonds (HBs), which it forms are typically much weaker than those of the hydroxyl group, particularly for a sp3‐hybridized C. An NH3nucleophile is allowed to approach both the terminal methyl group and the hydroxyl of n‐butanol, so as to form either a CH··N or OH··N HB. Density functional theory calculations show that the latter is much stronger than the former. However, the strength of the CH··N HB can be amplified and approach much closer to that of OH··N by appropriate placement of suitable electron‐withdrawing and donating substituents on the butanol. The interaction energy of the CH··N HB reaches above 6–8 kcal mol−1in several cases, considerably larger than the prototype HB within the water dimer.more » « less
-
Abstract We report production rates of H2O and nine trace molecules (C2H6, CH4, H2CO, CH3OH, HCN, NH3, C2H2, OCS, and CO) in long-period comet C/2020 S3 (Erasmus) using the high-resolution, cross-dispersed infrared spectrograph (iSHELL) at the NASA Infrared Telescope Facility, on two pre-perihelion dates at heliocentric distancesRh= 0.49 and 0.52 au. Our molecular abundances with respect to simultaneously or contemporaneously measured H2O indicate that S3 is depleted in CH3OH compared to its mean abundance relative to H2O among the overall comet population (Oort Cloud comets and Jupiter-family comets combined), whereas the eight other measured species have near-average abundances relative to H2O. In addition, compared to comets observed atRh< 0.80 au at near-infrared wavelengths, S3 showed enhancement in the abundances of volatile species H2CO, NH3, and C2H2, indicating possible additional (distributed) sources in the coma for these volatile species. The spatial profiles of volatile species in S3 in different instrumental settings are dramatically different, which might suggest temporal variability in comet outgassing behavior between the nonsimultaneous measurements. The spatial distributions of simultaneously measured volatile species C2H6and CH4are nearly symmetric and closely track each other, while those of CO and HCN co-measured with H2O (using different instrument settings) are similar to each other and are asymmetric in the antisunward direction.more » « less
