Anharmonic modeling of the conformation-specific IR spectra of ethyl, n-propyl, and n-butylbenzene

Abstract

Conformation-specific UV-IR double resonance spectra are presented for ethyl, n-propyl, and n-butylbenzene. With the aid of a local mode Hamiltonian that includes the effects of stretch-scissor Fermi resonance, the spectra can be accurately modeled for specific conformers. These molecules allow for further development of a first principles method for calculating alkyl stretch spectra. Across all chain lengths, certain dihedral patterns impart particular spectral motifs at the quadratic level. However, the anharmonic contributions are consistent from molecule to molecule and conformer to conformer. This transferability of anharmonicities allows for the Hamiltonian to be constructed from only a harmonic frequency calculation, reducing the cost of the model. The phenyl ring alters the frequencies of the CH2 stretches by about 15 cm(-1) compared to their n-alkane counterparts in trans configurations. Conformational changes in the chain can lead to shifts in frequency of up to 30 cm(-1).

Anharmonic interstellar PAH molecules

“…The computational time required to compute such large and diverse molecular compounds becomes prohibitive quickly. In that respect, new methods using local mode coupling or providing a way to generalise the construction of the QFFs for PAHs are very appealing.…”

Identifying the first folded alkylbenzene via ultraviolet, infrared, and Raman spectroscopy of pentylbenzene through decylbenzene

“…As a result, theoretical IR spectra were calculated in a local mode basis that explicitly accounted for stretch–bend coupling, much as was done in the previous work on smaller chain alkylbenzenes. 6 For the alkyl chains, the model Hamiltonian was constructed from the Hessian matrix and dipole derivatives computed at the B3LYP-D3/6-311++G(d,p) level of theory at each conformer's local reoptimized geometry. The only difference between the Hamiltonian construction in this work and that of the shorter chain study is how the stretch–stretch couplings between CH stretches on different carbon atoms are treated.…”

“…Alkyl chains in this size regime possess complicated potential energy landscapes that develop with chain length in fascinating ways, especially in the presence of the aromatic π cloud, which can engage in dispersive attractions with the alkyl chain. 6 – 8 In the pure n -alkanes, the dihedral angles about each C–C single bond prefer trans (180°) over gauche (±60°) arrangements due to steric effects. 9 As a result, the all- trans conformations are lowest in energy up to n = 17 ± 1, 10 with each gauche ‘defect’ causing a destabilization of about 2.5 kJ mol –1 .…”

“…Every observed conformation places C(1) in-plane and C(2) nearly perpendicular to the plane of the ring, as it is in ethylbenzene. We designated this initial dihedral as perpendicular, or p, in the previous study on short chain alkylbenzenes, 6 so we begin numbering the dihedrals starting with the next dihedral (C ph –C(1)–C(2)–C(3)). Only the dihedrals that deviate from the out-of-plane all- trans structure will be specified.…”

“…An essential tool in interpreting the single-conformer IR spectra is the local mode Hamiltonian model developed in the Sibert group, and applied recently to ethyl, n -propyl, and n -butylbenzene. 6 This model explicitly takes into account the strong and pervasive stretch–bend Fermi resonances that make comparison with harmonic normal mode calculations nearly impossible. We use many aspects of the model from that work without change, making at most minor modifications that further refine it.…”