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1. Derivation and implementation of the optical rotation tensor for chiral crystals

   https://doi.org/10.1063/5.0130385  

   Balduf, Ty; Caricato, Marco (December 2022, The Journal of Chemical Physics)

   This paper reports the derivation and implementation of the electric dipole-magnetic dipole and electric dipole-electric quadrupole polarizability tensors at the density functional theory level with periodic boundary conditions (DFT-PBC). These tensors are combined to evaluate the Buckingham/Dunn tensor that describes the optical rotation (OR) in oriented chiral systems. We describe several aspects of the derivation of the equations and present test calculations that verify the correctness of the tensor formulation and their implementation. The results show that the full OR tensor is completely origin invariant as for molecules and that PBC calculations match molecular cluster calculations on 1D chains. A preliminary investigation on the choice of density functional, basis set, and gauge indicates a similar dependence as for molecules: the functional is the primary factor that determines the OR magnitude, followed by the basis set and to a much smaller extent the choice of gauge. However, diffuse functions may be problematic for PBC calculations even if they are necessary for the molecular case. A comparison with experimental data of OR for the tartaric acid crystal shows reasonable agreement given the level of theory employed. The development presented in this paper offers the opportunity to simulate the OR of chiral crystalline materials with general-purpose DFT-PBC methods, which, in turn, may help to understand the role of intermolecular interactions on this sensitive electronic property. 

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2. A molecular orbital selection approach for fast calculations of specific rotation with density functional theory

   https://doi.org/10.1002/chir.23158  

   Aharon, Tal; Caricato, Marco (December 2019, Chirality)

   Abstract In this work, we describe a simple approach to select the most important molecular orbitals (MOs) to compute the optical rotation tensor through linear response (LR) Kohn‐Sham density functional theory (KS‐DFT). Taking advantage of the iterative nature of the algorithms commonly used to solve the LR equations, we select the MOs with contributions to the guess perturbed density that are larger than a certain threshold and solve the LR equations with the selected MOs only. We propose two criteria for the selection, and two definitions of the selection threshold. We then test the approach with two functionals (B3LYP and CAM‐B3LYP) and two basis sets (aug‐cc‐pVDZ and aug‐cc‐pVTZ) on a set of 51 organic molecules with specific rotation spanning five orders of magnitude, 10⁰–10⁴deg (dm⁻¹(g/mL)⁻¹). We show that this approach indeed can provide very accurate values of specific rotation with estimated speedup that ranges from 2 to 8× with the most conservative selection criterion, and up to 20 to 30× with the intermediate criterion. 

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3. A Fermi resonance and a parallel-proton-transfer overtone in the Raman spectrum of linear centrosymmetric N4H+: A polarizability-driven first principles molecular dynamics study

   https://doi.org/10.1063/5.0119251  

   Omodemi, Oluwaseun; Revennaugh, Ramsay; Riley, Janiyah; Kaledin, Alexey L.; Kaledin, Martina (October 2022, The Journal of Chemical Physics)

   We present molecular dynamics (MD), polarizability driven MD (α-DMD), and pump–probe simulations of Raman spectra of the protonated nitrogen dimer N4H+, and some of its isotopologues, using the explicitly correlated coupled-cluster singles and doubles with perturbative triples [CCSD(T)]-F12b/aug-cc-pVTZ based potential energy surface in permutationally invariant polynomials (PIPs) of Yu et al. [J. Phys. Chem. A 119, 11623 (2015)] and a corresponding PIP-derived CCSD(T)/aug-cc-pVTZ-tr (N:spd, H:sp) polarizability tensor surface (PTS), the latter reported here for the first time. To represent the PTS in terms of a PIP basis, we utilize a recently described formulation for computing the polarizability using a many-body expansion in the orders of dipole–dipole interactions while generating a training set using a novel approach based on linear regression for potential energy distributions. The MD/α-DMD simulations reveal (i) a strong Raman activity at 260 and 2400 cm−1, corresponding to the symmetric N–N⋯H bend and symmetric N–N stretch modes, respectively; (ii) a very broad spectral region in the 500–2000 cm−1 range, assignable to the parallel N⋯H+⋯N proton transfer overtone; and (iii) the presence of a Fermi-like resonance in the Raman spectrum near 2400 cm−1 between the Σg+ N–N stretch fundamental and the Πu overtone corresponding to perpendicular N⋯H+⋯N proton transfer. 

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4. Isolated and solvated chiroptical behavior in conformationally flexible butanamines

   https://doi.org/10.1002/chir.23570  

   Lemler, Paul M.; Craft, Clayton L.; Pollok, Corina H.; Regan, Thomas P.; Vaccaro, Patrick H. (June 2023, Chirality)

   Abstract The nonresonant optical activity of two highly flexible aliphatic amines, (2R)‐3‐methyl‐2‐butanamine (R‐MBA) and (2R)‐(3,3)‐dimethyl‐2‐butanamine (R‐DMBA), has been probed under isolated and solvated conditions to examine the roles of conformational isomerism and to explore the influence of extrinsic perturbations. The optical rotatory dispersion (ORD) measured in six solvents presented uniformly negative rotatory powers over the 320–590 nm region, with the long‐wavelength magnitude of chiroptical response growing nearly monotonically as the dielectric constant of the surroundings diminished. The intrinsic specific optical rotation, (in deg dm⁻¹[g/mL]⁻¹), extracted for ambient vapor‐phase samples ofR‐MBA [−11.031(98) and −2.29 (11)] andR‐DMBA [−9.434 (72) and −1.350 (48)] at 355 and 633 nm were best reproduced by counterintuitive solvents of high polarity (yet low polarizability) like acetonitrile and methanol. Attempts to interpret observed spectral signatures quantitatively relied on the linear‐response frameworks of density‐functional theory (B3LYP, cam‐B3LYP, and dispersion‐corrected analogs) and coupled‐cluster theory (CCSD), with variants of the polarizable continuum model (PCM) deployed to account for the effects of implicit solvation. Building on the identification of several low‐lying equilibrium geometries (nine forR‐MBA and three forR‐DMBA), ensemble‐averaged ORD profiles were calculated atT = 300 K by means of the independent‐conformer ansatz, which enabled response properties predicted for the optimized structure of each isomer to be combined through Boltzmann‐weighted population fractions derived from corresponding relative internal‐energy or free‐energy values, the latter of which stemmed from composite CBS‐APNO and G4 analyses. Although reasonable accord between theory and experiment was realized for the isolated (vapor‐phase) species, the solution‐phase results were less satisfactory and tended to degrade progressively as the solvent polarity increased. These trends were attributed to solvent‐mediated changes in structural parameters and energy metrics for the transition states that separate and putatively isolate the equilibrium conformations supported by the ground electronic potential‐energy surface, with the resulting displacement of barrier locations and/or decrease of barrier heights compromising the underlying premise of the independent‐conformer ansatz. 

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5. A polarizability driven ab initio molecular dynamics approach to stimulating Raman activity: Application to C ₂₀

   https://doi.org/10.1080/00268976.2021.1939453  

   Pierre-Jacques, Dominick; Tyler, Ciara; Dyke, Jason; Kaledin, Alexey L.; Kaledin, Martina (June 2021, Molecular Physics)

   null (Ed.)

   We describe a novel variant of the driven molecular dynamics (DMD) method derived for probing Raman active vibrations. The method is an extension of the conventional alpha-DMD formulation for simulating IR activity by means of coupling an oscillating electric field to the molecule’s dipole moment, miu, and inducing absorption of energy via tuning the field to a resonant frequency. In the present work, we modify the above prescription to invoke Raman activity by coupling two electric fields, i.e., a “Pump” photon of frequency wP and a Stokes photon of frequency wS to the molecule’s polarizability tensor, alpha, with the difference in the frequencies of the two photons w = wP - wS corresponding to the Stokes Raman shift. If a particular w is close to a Raman active vibrational frequency, energy absorption by the molecule ensues. Varying w over the desired frequency range allows identifying and assigning all Raman active vibrational modes, including anharmonic corrections, in the range by means of trajectory analysis. We show that only one element of the full polarizability tensor, and its nuclear derivative, is needed for an alpha-DMD trajectory, making this method well suited for ab initio dynamics implementation. Numerical results using first-principles calculations are presented and discussed for the vibrational fundamentals, combination bands, overtones of H2O, CH4, and the C20 fullerene. 

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