Bryan R. Henry

CALCULATION OF VIBRATIONAL FUNDAMENTAL AND OVERTONE BAND INTENSITIES OF H2O

Vibrational intensities are calculated for the fundamental and overtone transitions of H2O up to approximately 18u2009000 cm−1. The intensities are determined from a dipole moment function expanded in the three internal bond coordinates. The expansion coefficients are computed ab initio at the second‐order Mo/ller–Plesset level of theory with a 6‐311G** basis set. Vibrational wave functions are calculated either from a three‐dimensional harmonically coupled anharmonic oscillator (HCAO) model which uses Morse oscillators to represent both the stretches and the bend of H2O, or from a variational calculation employing the best available potential energy surface and an exact kinetic energy operator. To obtain the most meaningful vibrational intensities we define dipole moment components using the Eckart embedding. Both the HCAO and the variational intensities agree quite well with the experimental results, which span eight orders of magnitude. From the calculations we predict that it may be possible to detect as ...

Intensities in local mode overtone spectra: Propane

The gas phase vibrational overtone spectrum of propane is measured using conventional near infrared (NIR) spectroscopy for the ΔvCH=2–5 regions and intracavity dye laser photoacoustic spectroscopy (IDL‐PAS) for the ΔvCH=5 and 6 regions. The peaks are assigned in terms of the local mode model. Experimental oscillator strengths are compared to values calculated for the CH‐stretching components of the spectrum. The calculations use a harmonically coupled, anharmonic oscillator local mode model to obtain the vibrational wave functions, and ab initio MO calculations at the SCF level with a 6‐31G* basis set to obtain the dipole moment function. The importance of intermanifold coupling is explored. The calculations can account for the fall‐off in intensity with increasing v, and can give a reasonably quantitative account of the relative intensities of the individual peaks within a given vibrational manifold. The questions of the relative intensities of primary and secondary CH bonds, and of the relative intensit...

The relative intensity contributions of axial and equatorial CH bonds in the local mode overtone spectra of cyclohexane

Vapor phase room temperature overtone spectra of cyclohexane are measured with conventional near infrared spectroscopy for the lower overtones, ΔvCH=2–4, and with intracavity dye laser photoacoustic spectroscopy for the higher overtones, ΔvCH=5–7. The relative intensity of the axial to equatorial peak is explained in terms of the local mode model of harmonically coupled anharmonic oscillators, where all modes but the two CH‐stretching modes are neglected. The dipole moment function is expanded in the two CH‐stretching coordinates, where the expansion coefficients are determined from ab initio molecular orbital calculations. This simple calculation, which contains no adjustable parameters, can account very well for the observed relative intensity of axial and equatorial pure local mode peaks in the overtone spectra from ΔvCH=3–7. We also investigate different ways of obtaining the dipole moment derivatives, and find that a fourth order series expansion around the equilibrium geometry is a good approximatio...

Calculation of vibrational (J=0) excitation energies and band intensities of formaldehyde using the recursive residue generation method

We use the recursive residue generation method (RRGM) with an exact kinetic energy operator to calculate vibrational excitation energies and band intensities for formaldehyde. The basis is a product of one‐dimensional potential optimized discrete variable representation (PO‐DVR) functions for each coordinate. We exploit the symmetry by using symmetry adapted basis functions obtained by taking linear combinations of PO‐DVR functions. Our largest basis set consists of 798 600 functions (per symmetry block). The Lanczos tridiagonal representation of the Hamiltonian is generated iteratively (without constructing matrix elements explicitly) by sequential transformations. We determine a six‐dimensional dipole moment function from the ab initio dipole moment values computed at the QCISD level with a 6‐311++G(d,p) basis set. We converged all A1, B2 and B1 vibrational states up to the combination band with two quanta in the C–O stretch and one quantum in a C–H stretch at about 6 350 cm−1 above zero point energy. W...

Intensities in local mode overtone spectra of dimethyl ether and acetone

Gas phase overtone spectra of dimethyl ether (ΔvCH=2–5) and acetone (ΔvCH=3) are measured using conventional near infrared (NIR) spectroscopy. Intracavity dye laser photoacoustic spectroscopy (IDL‐PAS) has been used to measure the ΔvCH=5–7 gas phase spectra of both dimethyl ether and acetone. Oscillator strengths are calculated using a harmonically coupled anharmonic oscillator local mode description to obtain the vibrational wave functions and ab initio molecular orbital (MO) calculations to obtain the dipole moment function. The calculations, which use no adjustable parameters, can account for the magnitude of the intensities and for the falloff in intensity with increasing v, for both molecules. It can also account reasonably well for the relative intensity of various peaks within a given vibrational manifold and for the relative intensity in the spectra of these two molecules and of propane.

Absolute Intensities of NH-Stretching Transitions in Dimethylamine and Pyrrole

Vibrational spectra of vapor-phase dimethylamine (DMA) and pyrrole have been recorded in the 1000 to 13000 cm(-1) region using long path conventional spectroscopy techniques. We have focused on the absolute intensities of the NH-stretching fundamental and overtone transitions; Δν(NH) = 1-4 regions for DMA and the Δν(NH) = 1-3 regions for pyrrole. In the Δν(NH) = 1-3 regions for DMA, evidence of tunneling splitting associated with the NH-wagging mode is observed. For DMA, the fundamental NH-stretching transition intensity is weaker than the first NH-stretching overtone. Also, the fundamental NH-stretching transition in DMA is much weaker than the fundamental transition in pyrrole. We have used an anharmonic oscillator local mode model with ab initio calculated local mode parameters and dipole moment functions at the CCSD(T)/aug-cc-pVTZ level to calculate the NH-stretching intensities and explain this intensity anomaly in DMA.

Intensities of CH‐ and CD‐stretching overtones in 1,3‐butadiene and 1,3‐butadiene‐d6

Gas phase vibrational overtone spectra of 1,3‐butadiene are recorded in the ΔvCH=2–6 regions by conventional near infrared–visible spectroscopy, and in the ΔvCH=4–7 regions by intracavity dye/titanium:sapphire, laser photoacoustic spectroscopy (ICL‐PAS). Gas phase vibrational overtone spectra of 1,3‐butadiene‐d6 are recorded in the ΔvCD=2–5 regions with conventional spectroscopy and in the ΔvCD=5–8 regions by ICL‐PAS. Oscillator strengths are calculated from wave functions that are obtained from a harmonically coupled anharmonic oscillator (HCAO) local mode model and from a dipole moment function that is obtained from ab initio calculations. The experimental oscillator strengths are compared to the values that are calculated for both the CH‐ and CD‐stretching components of the spectrum. Our simple calculations, which contain no adjustable parameters, are in very good agreement with the relative intensities of the peaks corresponding to the three different CH oscillators in 1,3‐butadiene. As expected, the ...

Ab initio calculation of dipole moment functions: application to vibrational band intensities of H2O

The dipole moment function of H2O has been calculated at different levels of ab initio theory. The ab initio calculations range from Hartree-Fock to perturbative, limited configuration interaction, and coupled cluster methods, and cover a wide range of standard basis sets. The dipole moment function is taken as a series expansion in the bond coordinates, with expansion coefficients found from the ab initio results. These different dipole moment functions are used to calculate fundamental and overtone vibrational band intensities of H2O. A simple three-dimensional harmonically coupled anharmonic oscillator local mode description is employed to obtain the vibrational energies and wave-functions. The results indicate that useful vibrational band intensities can be obtained from a limited ab initio approach. Fundamental intensities are more sensitive to electron correlation than overtone intensities. This provides an insight into the quality of dipole moment function necessary to calculate reasonable values f...

Calculated intensity in the local mode overtone spectra of hydrogen peroxide

Calculations of OH stretching vibrational intensities for hydrogen peroxide are performed with a harmonically coupled anharmonic local mode model for the OH stretching vibrational wave functions and ab initio calculations to obtain the dipole moment function. The ab initio calculations are carried out at three different levels: Self‐consistent field (SCF), second‐order Mo/ller–Plesset perturbation theory (MP2), and configuration interaction including all single and double excitations (CISD), all with a 6–31G* basis set, to yield the dipole moment function. From these results, total oscillator strengths are calculated for the three different types of rotational bands in a given OH stretching transition. The calculated overtone intensities for ΔvOH=2–5 are not sensitive to the particular dipole moment function. However, a significant decrease in intensity occurs for the fundamental transition in going from the molecular orbital to the correlated level calculations. Our calculated results are compared to nea...

Intensity of CH‐ and NH‐stretching transitions in the overtone spectra of cyclopropylamine

Vapor phase room temperature overtone spectra of cyclopropylamine are measured both with conventional absorption spectroscopy for the lower overtones and with intracavity laser photoacoustic spectroscopy for the higher overtones. Spectra are recorded in both the CH‐stretching (ΔvCH=2–7) and NH‐stretching (ΔvNH=2–6) regions. The relative intensities of XH‐stretching peaks are explained in terms of the local mode model of harmonically coupled anharmonic oscillators, where all modes but the XH‐stretching modes are neglected. The dipole moment function is expanded in the XH‐stretching coordinates, with the expansion coefficients determined from ab initio molecular orbital calculations. The high number of peaks throughout the spectra makes assignment difficult, but allows the overlap of spectra from different experimental conditions. Thus it is possible to approximately determine the absolute intensities for the higher overtone spectra that are obtained from the laser experiments. The simple intensity calculat...