Infrared Spectra of Dimethylquinolines in the Gas Phase: Experiment and Theory

Abstract

Infrared spectra of atmospherically important dimethylquinolines (DMQs), namely 2,4-DMQ, 2,6-DMQ, 2,7-DMQ, and 2,8-DMQ in the gas phase at 80 degrees C were recorded using a long variable path-length cell. DFT calculations were carried out to assign the bands in the experimentally observed spectra at the B3LYP/6-31G* level of theory. The spectral assignments particularly for the C-H stretching modes could not be made unambiguously using calculated anharmonic or scaled harmonic frequencies. To resolve this problem, a scaled force field method of assignment was used. Assignment of fundamental modes was confirmed by potential energy distributions (PEDs) of the normal modes derived by the scaled force fields using a modified version of the UMAT program in the QCPE package. We demonstrate that for large molecules such as the DMQs, the scaling of the force field is more effective in arriving at the correct assignment of the fundamentals for a quantitative vibrational analysis. An error analysis of the mean deviation of the calculated harmonic, anharmonic, and force field fitted frequencies from the observed frequency provides strong evidence for the correctness of the assignment.

Conformational Stability and Intramolecular Hydrogen Bonding in 1,2-Ethanediol and 1,4-Butanediol

“…Details of the experimental set-up have been described elsewhere. 39,40 In brief, the gas phase mid-IR spectra were collected using a Vertex-70 FT-IR spectrometer (Bruker Optics, Germany) coupled with a 6 meter heated multi-pass long-path gas cell (Model 7.2-V, REFLEX Analytical Corporation). The cell body is a cylinder of borosilicate glass of length 20 cm with an inside diameter of 6 cm.…”

“…The samples 1,2-ethanediol (HOCH 2 CH 2 OH, LR grade, Ranbaxy) and 1,4-butanediol (HOCH 2 CH 2 CH 2 CH 2 OH, 98%, Merck) were used following several cycles of freeze–pump–thaw to remove dissolved gases. Details of the experimental setup have been described elsewhere. , In brief, the gas-phase mid-IR spectra were collected using a Vertex-70 FT-IR spectrometer (Bruker Optics, Germany) coupled with a 6 m heated multipass long-path gas cell (Model 7.2-V, REFLEX Analytical Corporation). The cell body is a cylinder of borosilicate glass of length 20 cm with an inside diameter of 6 cm.…”

Continuous Scan and Repetitive Mode FT-IR Spectroscopy and Its Application in Isomeric Identification, Conformational Analysis and Photochemistry

“…The amount of sample (reactant) decay and formation of new products can be measured from the integrated band area of the observed infrared spectra using Eq. ( 6) [25], which is basically the modified Eq. ( 5).…”

A spectroscopic method for monitoring photochemical reactions in the gas phase

“…We measured the decay of sample and formation of products from the integrated band area of the observed infrared spectra using the expression given in ref [10] , in which, τ i is the integrated absorbance (in cm −1 ), (in km mol −1 ) is the absorption coefficient, l (in cm) is the optical path length, and T (298 K) is the experimental temperature. Typically, we chose 2 - 3 absorption lines of a species and averaged the derived partial pressure.…”