Jingping Peng

Piezoelectric detection of vibrational overtones at cryogenic temperatures

A resonant optoacoustic technique is described using continuous wave laser excitation and piezoelectric detection of overtone absorptions of liquefied gases at cryogenic temperatures. This technique uses a piezoelectric detector directly attached to a low temperature cell inside a cryostat. The liquefied sample inside the cell is vibrationally excited in the visible region with a continuous wave dye laser. The laser is modulated around 100 kHz with an acousto‐optic modulator. The modulation frequency corresponds to an acoustic resonance frequency of the cylindrical cell which increases the sensitivity of the detection system. The pressure wave detected by the piezoelectric transducer is sent to a lock‐in amplifier and processed with a microcomputer. This sensitive technique is used to obtain the visible spectra of pure liquid methane and ethane. Spectra of the fourth overtone of the C‐H stretch vibration of methane, in solution with liquid argon at 94 K, and in solution with liquid nitrogen at 84 K, are a...

Overtone spectroscopy of isobutane at cryogenic temperatures

Abstract The spectra of the fundamental and overtones of the CH stretches of (CH 3 ) 3 CH have been measured in liquid argon solutions at 90 K and for the pure liquid sample at 135 K. Absorptions in the visible were obtained with a low temperature cell and a resonant continuous wave laser technique with acoustic detection. Absorptions in the IR and near-IR were observed with a Fourier transform spectrophotometer. Comparison is made between the absorption bands in gas phase, liquid argon solution, and liquid phase isobutane. The spectra of isobutane in solution show improved resolution of the vibrational bands with respect to the room temperature gas phase bands and the pure liquid bands at 135 K. To interpret the experimental results, overtone transitions are described in terms of the local mode model. A harmonically coupled anharmonic oscillator (HCAO) model was used to determine the overtone energy levels and assign the absorption bands to vibrational transitions. Ab initio molecular orbital calculations of geometries and vibrational frequencies were also performed.

Vibrational spectroscopy of CH bonds of methane and tetramethylsilane in liquid argon solutions

Abstract The liquid phase spectra of the fundamental and first overtone (Δ v = 1, 2) around the CH stretch of methane and tetramethylsilane in liquid argon solutions have been measured using Fourier transform IR and near-IR techniques. The fourth overtone (Δ v = 5) of the CH stretch has been measured in methane pure at 116 K and in liquid argon solutions at 100 K using laser excitation with acoustic detection. The results indicate that absorption bands, which normally present extensive inhomogeneous congestion, could be simplified by studying the absorptions in diluted solutions at low temperatures with inert liquefied gases as solvents.

Vibrational ab initio calculations and spectra of C–H bonds of trimethylboron

Visible laser photoacoustic and near‐IR spectra of the overtones of the C–H stretches of (CH3)3B in the gas phase are reported. Two bands are assigned to nonequivalent methyl C–H bonds. The interaction of an empty 2p orbital of the boron atom with the C–H bonds of a methyl group changes the strength of the C–H bonds during the internal rotation. The most intense, higher energy absorption band in each overtone region is assigned to the CH bonds in the molecular plane (C–H∥) and the least intense, lower energy absorption band to the CH bonds out of the molecular plane (C–H⊥). To interpret the experimental results, overtone transitions are described in terms of the local mode model. A harmonically coupled anharmonic oscillator (HCAO) model was used to determine the overtone energy levels and assign the absorption bands to particular transitions. Ab initio molecular orbital calculations were also performed. Equilibrium geometries, vibrational frequencies, and infrared intensities were calculated at the Hartre...

Infrared absorptions and band widths of dilute solutions of CF3H in liquid Ar, N2, and Xe

The spectra of fluoroform (CF3H) in the solvents Ar, N2, and Xe have been obtained in the fundamental region (400–4000cm−1) using a low temperature cryostat and a Fourier transform infrared spectrophotometer. Ab initio calculations at the HF/6-31G∗ level have been performed to obtain the calculated vibrational frequencies of the isolated CF3H molecule and CF3H in the presence of the solvents (Ar, N2, and Xe). Comparison of the frequency shifts of CF2H in solution with respect to the gas phase frequencies is made for the experimental and theoretical results. Lorentzian functions were used to fit the bands and obtain the wavenumber at the peak absorbance and the vibrational band widths. An analysis of the dynamics of relaxation has been made based on the infrared time correlation functions for three of the fundamental modes (ν1, ν3, and ν4). Bandwidths, band moments, and relaxation times were obtained by appropriate fitting of the experimental correlation functions to theoretical models. In liquid argon, th...

Vibrational spectroscopy of nonequivalent CH bonds in liquid cis- and trans-3-hexene

Abstract The liquid phase vibrational fundamental and overtone spectra (Δν = 1–4) of nonequivalent CH bonds of cis -3-hexene and trans -3-hexene have been recorded using FTIR and near-IR spectroscopy. A technique which uses a piezoelectric detector, lock-in amplification, and a continuous wave dye laser modulated at 80 kHz with an acousto-optic modulator, has been used to detect the fourth (Δν = 5) and fifth (Δν = 6) overtones. The overtone bands have been computer deconvoluted to obtain information with respect to peak position and bandwidth of the overtone absorptions. Transitions corresponding to different CH bonds, olefinic (CH), primary (CH 3 ) and secondary (CH 2 ) are assigned using the local mode model. Other bands are assigned as local mode—normal mode combination bands. Local mode frequencies ω i and anharmonicities X ii are obtained for all the nonequivalent CH bonds. The results obtained for cis -3-hexene are used to interpret the Δν = 3 and 4 CH overtone spectra of cis -1,4-polybutadiene.

Cis- and trans-3-hexene: infrared spectrum in liquid argon solution, ab initio calculations of equilibrium geometry, normal coordinate analysis, and vibrational assignments

Abstract The infrared spectra of cis -3-hexene and trans -3-hexene dissolved in liquid argon have been obtained at temperatures from 93 to 120 K. The absorptions were observed with a low-temperature cell and a Fourier transform infrared spectrophotometer. Ab initio molecular orbital calculations were performed to obtain the equilibrium geometry, vibrational frequencies, force fields, and infrared intensities. The calculations were done at the Hartree-Fock level using 6-31G basis set. The Cartesian force fields from ab initio calculations have been converted to the force field in symmetry coordinates. The scale factors of ab initio calculated force fields were determined. Normal coordinate calculations were performed using a scaled quantum mechanical (SQM) force field. Vibrational normal modes calculated for the lowest energy rotamers of cis - and trans -3-hexene have been assigned to infrared absorption bands observed in liquid argon solution. The assignments were based on calculated frequencies and potential energy distributions. The equilibrium geometries of the two lowest energy rotamers (symmetry C 2 and C s ) of cis -3-hexene and of the three lowest energy rotamers (symmetry C i , C 2 , and C 1 ) of trans -3-hexene were calculated. Variable temperature studies of the infrared spectrum of cis - and trans -3-hexenes dissolved in liquid argon were done to obtain the ΔH of conversion between the rotamers C 2 and C s of cis -3-hexene and between the rotamers C i , C 2 , and C 1 of trans -3-hexene.

Infrared spectroscopy of 2-methyl-2-butene in liquid argon solution and ab initio calculated vibrational frequencies

Abstract The infrared spectrum of 2-methyl-2-butene dissolved in liquid argon has been measured at 90 K. The absorption was obtained with a low-temperature cell and a Fourier transform spectrophotometer. The solubility of 2-methyl-2-butene in liquid argon was measured. Ab initio molecular orbital calculations were performed to obtain the equilibrium geometry, vibrational frequencies, and infrared intensities. The calculations were done at the Hartree-Fock level using the 6-31G ∗ split-valence basis set. A normal coordinate analysis was performed using a scaled force field determined from the 6-31G ∗ basis set. The potential energy distribution was also obtained.