### Abstract

Using a recently developed recursive scheme for the calculation of high-order geometric derivatives of frequency-dependent molecular properties [Ringholm et al., J. Comp. Chem., 2014, 35, 622], we present the first analytic calculations of anharmonic infrared (IR) and Raman spectra including anharmonicity both in the vibrational frequencies and in the IR and Raman intensities. In the case of anharmonic corrections to the Raman intensities, this involves the calculation of fifth-order energy derivatives - that is, the third-order geometric derivatives of the frequency-dependent polarizability. The approach is applicable to both Hartree-Fock and Kohn-Sham density functional theory. Using generalized vibrational perturbation theory to second order, we have calculated the anharmonic infrared and Raman spectra of the non- and partially deuterated isotopomers of nitromethane, where the inclusion of anharmonic effects introduces combination and overtone bands that are observed in the experimental spectra. For the major features of the spectra, the inclusion of anharmonicities in the calculation of the vibrational frequencies is more important than anharmonic effects in the calculated infrared and Raman intensities. Using methanimine as a trial system, we demonstrate that the analytic approach avoids errors in the calculated spectra that may arise if numerical differentiation schemes are used.

Original language | English |
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Pages (from-to) | 4201-4215 |

Number of pages | 15 |

Journal | Physical Chemistry Chemical Physics |

Volume | 18 |

Issue number | 5 |

Early online date | 7 Jan 2016 |

DOIs | |

Publication status | Published - 2016 |

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*Physical Chemistry Chemical Physics*,

*18*(5), 4201-4215. https://doi.org/10.1039/C5CP06657C, https://doi.org/10.1039/c5cp06657c

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*Physical Chemistry Chemical Physics*, vol. 18, no. 5, pp. 4201-4215. https://doi.org/10.1039/C5CP06657C, https://doi.org/10.1039/c5cp06657c

**Analytic calculations of anharmonic infrared and Raman vibrational spectra.** / Cornaton, Yann; Ringholm, Magnus; Louant, Orian; Ruud, Kenneth.

Research output: Contribution to journal › Article

TY - JOUR

T1 - Analytic calculations of anharmonic infrared and Raman vibrational spectra

AU - Cornaton, Yann

AU - Ringholm, Magnus

AU - Louant, Orian

AU - Ruud, Kenneth

PY - 2016

Y1 - 2016

N2 - Using a recently developed recursive scheme for the calculation of high-order geometric derivatives of frequency-dependent molecular properties [Ringholm et al., J. Comp. Chem., 2014, 35, 622], we present the first analytic calculations of anharmonic infrared (IR) and Raman spectra including anharmonicity both in the vibrational frequencies and in the IR and Raman intensities. In the case of anharmonic corrections to the Raman intensities, this involves the calculation of fifth-order energy derivatives - that is, the third-order geometric derivatives of the frequency-dependent polarizability. The approach is applicable to both Hartree-Fock and Kohn-Sham density functional theory. Using generalized vibrational perturbation theory to second order, we have calculated the anharmonic infrared and Raman spectra of the non- and partially deuterated isotopomers of nitromethane, where the inclusion of anharmonic effects introduces combination and overtone bands that are observed in the experimental spectra. For the major features of the spectra, the inclusion of anharmonicities in the calculation of the vibrational frequencies is more important than anharmonic effects in the calculated infrared and Raman intensities. Using methanimine as a trial system, we demonstrate that the analytic approach avoids errors in the calculated spectra that may arise if numerical differentiation schemes are used.

AB - Using a recently developed recursive scheme for the calculation of high-order geometric derivatives of frequency-dependent molecular properties [Ringholm et al., J. Comp. Chem., 2014, 35, 622], we present the first analytic calculations of anharmonic infrared (IR) and Raman spectra including anharmonicity both in the vibrational frequencies and in the IR and Raman intensities. In the case of anharmonic corrections to the Raman intensities, this involves the calculation of fifth-order energy derivatives - that is, the third-order geometric derivatives of the frequency-dependent polarizability. The approach is applicable to both Hartree-Fock and Kohn-Sham density functional theory. Using generalized vibrational perturbation theory to second order, we have calculated the anharmonic infrared and Raman spectra of the non- and partially deuterated isotopomers of nitromethane, where the inclusion of anharmonic effects introduces combination and overtone bands that are observed in the experimental spectra. For the major features of the spectra, the inclusion of anharmonicities in the calculation of the vibrational frequencies is more important than anharmonic effects in the calculated infrared and Raman intensities. Using methanimine as a trial system, we demonstrate that the analytic approach avoids errors in the calculated spectra that may arise if numerical differentiation schemes are used.

UR - http://www.scopus.com/inward/record.url?scp=85000623113&partnerID=8YFLogxK

U2 - 10.1039/C5CP06657C

DO - 10.1039/C5CP06657C

M3 - Article

VL - 18

SP - 4201

EP - 4215

JO - Physical chemistry chemical physics : PCCP

JF - Physical chemistry chemical physics : PCCP

SN - 1463-9076

IS - 5

ER -