Comparison of Calculated Normal Mode Molecular Vibrations with Experimental Gas-Phase Infrared Spectroscopy
type: CreativeWork
description: Computational vibrational spectroscopy serves as an important tool in the interpretation of experimental infrared (IR) spectra. Analysis of computational results provides a perspective over broader wavelength ranges and at higher precision. Although there are issues regarding accuracy, this can be approximated by using a scaling factor. High-resolution gas-phase FTIR spectroscopy at a resolution of 0.125 cm-1 can partially resolve rovibrational transitions in the P, Q, and R bands and therefore identify fundamental frequencies with approximately 1 cm-1 precision. This research has compared high-resolution gas-phase FTIR absorption peaks to calculated vibrational frequencies. In the calculation of normal mode frequencies, reliability, feasibility, and ease of interpretation are still a matter of concern due to the interplay of several factors (time, accuracy, and memory requirement of computer hardware). Calculations were performed using different methods (theories and basis sets), such as, B3LYP-D/cc-pVDZ, MP2-cc-pVDZ, CCSD/cc-pVDZ, CCSD(T)/aug-cc-pVDZ, CCSD(T)/aug-cc-pVTZ, CCSD/cc-pVDZ and CCSD/cc-pVTZ on different diatomic and polyatomic molecules using the ab initio computational software package, Psi4. H2, C2, HCl and CO required 15-20 minutes to get the output file with ± 0 to 8 cm-1 variations compared to experimental values. Couple cluster theory (CC) yields quite accurate calculations for diatomic molecules. Whereas, with polyatomic H2O, CH4 and CH2O required a time of 45 minutes to 1 hour with variation of ± 0 to 20 cm-1. For H2O couple cluster theory is best. For CH4 and CH2O density functional theory (DFT) provides the nearly similar number to experimental frequencies. The molecules, CH2O2, CH3COOH and C3H6O, required up to 48 hours due to the number of electrons (N) in each molecule and the number of calculations needed for a particular theory, for example MP2 scales as O(N5) and CCSD(T) scales as O(N7). Vibrational frequencies were calculated using only…
abstract: Computational vibrational spectroscopy serves as an important tool in the interpretation of experimental infrared (IR) spectra. Analysis of computational results provides a perspective over broader wavelength ranges and at higher precision. Although there are issues regarding accuracy, this can be approximated by using a scaling factor. High-resolution gas-phase FTIR spectroscopy at a resolution of 0.125 cm-1 can partially resolve rovibrational transitions in the P, Q, and R bands and therefore identify fundamental frequencies with approximately 1 cm-1 precision. This research has compared high-resolution gas-phase FTIR absorption peaks to calculated vibrational frequencies. In the calculation of normal mode frequencies, reliability, feasibility, and ease of interpretation are still a matter of concern due to the interplay of several factors (time, accuracy, and memory requirement of computer hardware). Calculations were performed using different methods (theories and basis sets), such as, B3LYP-D/cc-pVDZ, MP2-cc-pVDZ, CCSD/cc-pVDZ, CCSD(T)/aug-cc-pVDZ, CCSD(T)/aug-cc-pVTZ, CCSD/cc-pVDZ and CCSD/cc-pVTZ on different diatomic and polyatomic molecules using the ab initio computational software package, Psi4. H2, C2, HCl and CO required 15-20 minutes to get the output file with ± 0 to 8 cm-1 variations compared to experimental values. Couple cluster theory (CC) yields quite accurate calculations for diatomic molecules. Whereas, with polyatomic H2O, CH4 and CH2O required a time of 45 minutes to 1 hour with variation of ± 0 to 20 cm-1. For H2O couple cluster theory is best. For CH4 and CH2O density functional theory (DFT) provides the nearly similar number to experimental frequencies. The molecules, CH2O2, CH3COOH and C3H6O, required up to 48 hours due to the number of electrons (N) in each molecule and the number of calculations needed for a particular theory, for example MP2 scales as O(N5) and CCSD(T) scales as O(N7). Vibrational frequencies were calculated using only…
author: Sutar, Anila Renis
keywords: FTIR, Coupled cluster theory, Psi4, Density functional theory, Vibrational normal modes, Physical Chemistry
date published: 2021-01-01T08:00:00Z
date modified: 2026-07-24T01:57:53Z
provider: City University of New York - City College
publisher: Networked Digital Library of Theses and Dissertations
type: WebPage
source organization: City University of New York - City College