Absolute Infrared Absorption Intensities Research Articles

Ab initio prediction of the infrared absorption spectrum of the C2Br radical

The first three electronic states of the C2Br radical, correlating at linear geometries with 2Σ+ and 2Π states, have been studied ab initio, using Multi Reference Configuration Interaction techniques. The electronic ground state is found to have a bent equilibrium geometry, RCC=1.2621Å, R CBr=1.7967Å, ∠ CCBr=156.1°, with a very low barrier to linearity. Similarly to the valence isoelectronic radicals C2F and C2Cl, this anomalous behaviour is attributed to a strong three-state non-adiabatic electronic interaction. The Σ ,Π1/2,Π3/2 vibronic energy levels and their absolute infrared absorption intensities at a temperature of 5 K have been calculated for the 12 C12 C79Br isotopomer, to an upper limit of 2000 cm−1, using ab initio diabatic potential energy and dipole moment surfaces and a recently developed variational method.

Optical constant, dielectric constant and molar polarizability spectra of liquid hexane between 4000 and 400 cm − 1 at 25 °C

This study forms part of a larger study on absolute infrared absorption intensities of organic liquids. Previously, the optical constants of a series of benzene derivatives (benzene, benzene-d 6, benzene-d 1, chlorobenzene, bromobenzene, bromobenzene-d 5, toluene, toluene-d 8, hexafluorobenzene, ethylbenzene, fluorobenzene) were reported. In the current study, the infrared absorption spectra of liquid hexane were recorded at 25 °C from 4000 to 400 cm −1 using transmission cells of 10 to 4251 μm. The optical constants (complex refractive indices), complex dielectric constants, and complex molar polarizability spectrum of liquid hexane were determined.

Infrared intensities of liquids. Part XXIII. Infrared optical constants and integrated intensities of liquid benzene-d1 at 25°C

This paper presents the first absolute infrared absorption intensities of liquid benzene-d1, C6H5D, at 25°C. It also presents what, surprisingly, seems to be the first complete assignment of an infrared spectrum of liquid benzene-d1 recorded with post-1950 instrumentation. The spectra of the real and imaginary refractive indices are given as graphs and tables between 6200 and 500cm−1, and a table is given of the peak wavenumbers, absolute integrated intensities and vibrational assignments between 4800 and 500cm−1. Errors in the imaginary refractive index, k, values are estimated to be 5–7% for peaks and 5–20% for the baseline between 6200 and 4700cm−1, 0.3–2.0% for both peaks and baseline between 4700 and 825cm−1, 3–4% for both peaks and baseline between 825 and 620cm−1, and 40–50% between 620 and 500cm−1 where the very strong peak near 600cm−1 was too intense for us to measure accurately. Errors in the real refractive indices, n, are estimated to be 0.25% at 8000cm−1 increasing to 0.5% near 800cm−1 and, due to the uncertain intensity of the peak near 600cm−1, to range up to 10% between 710 and 500cm−1. The refractive index spectra were converted to spectra of the real and imaginary dielectric constants, ϵ′ and ϵ″, the molar absorption coefficient, Em, and the real and imaginary molar polarizabilities under the Lorentz local field, α′m and α″m. The peak heights and wavenumbers in the spectra of the different absorption quantities are compared for the most intense bands. Integrated intensities were determined as Cj, the area under bands in the ν̃α″m spectrum, for all bands between 4800 and 500cm−1. The contributions from the different bands were separated by fitting the spectrum with classical damped harmonic oscillator bands. The estimated errors in the integrated intensities range from 2 to 10% for most bands, although they may reach 100% for very weak bands and shoulders. The integrated intensities of the fundamentals and the corresponding transition dipole moments are summarized and compared with literature values for the gas. Crawford's F-sum rule shows that the measured integrated intensities of C6H5D are nicely consistent with those reported recently for C6H6 and C6D6. The total integrated intensity of the first overtone of the CH stretches is ∼20 times smaller than that of the fundamentals.

Accurate ab initio near-equilibrium potential energy and dipole moment functions of the ground electronic state of ozone

We report a highly correlated multireference configuration interaction calculation of the near-equilibrium potential energy surface of ozone using a large correlation consistent basis set. Three-dimensional analytical expressions are obtained for the potential energy and dipole moment functions using least-squares fits to ab initio points near the C2v equilibrium geometry. Low-lying vibrational band origins of O316 and some of its isotopic variants are calculated using the ab initio potential energy function. The calculated fundamental frequencies for the symmetric stretching and bending vibrations are within about 3 cm−1 of the observed values, while that for the antisymmetric stretch deviates from experiment by about 13 cm−1. The agreement with experiment can be significantly improved if the ab initio potential energy function is scaled in the antisymmetric stretching coordinate. Absolute infrared absorption intensities are also calculated using ab initio electric dipole moment functions and in good agreement with the available experimental data.

Accurate ab initio near-equilibrium potential energy and dipole moment functions of the X 2B1 and first excited A22 electronic states of OClO and OBrO

Using highly correlated multireference configuration interaction wave functions with large correlation consistent basis sets, three-dimensional near-equilibrium potential energy functions (PEFs) have been calculated for the X 2B1 and first excited A22 electronic states of the atmospherically important OClO and OBrO radicals. The analytical PEFs have been used in perturbational and variational calculations of the anharmonic spectroscopic constants and vibrational spectra of both species. Excellent agreement with the available experimental data are observed for both species and electronic states, e.g., the vibrational fundamental frequencies in the ground electronic states are reproduced to within about 5 cm−1. For the A 2A2 state of OClO, it is demonstrated that the anomolously strong intensity of the ν3 mode in the UV absorption spectrum is due to strong anharmonic coupling between the stretching vibrations and not to a double minimum in the potential. Three-dimensional electric dipole moment functions have also been calculated for the ground electronic states of both species. These were used to calculate accurate absolute infrared absorption intensities for the fundamentals and low-lying overtones and combination bands of both species.

Infrared intensities of liquids XXI: integrated absorption intensities of CH 3OH, CH 3OD, CD 3OH and CD 3OD and dipole moment derivatives of methanol

This paper presents the analysis of the complete set of vibrational intensities of four isotopomers of methanol. The absolute infrared absorption intensities of liquid methanol in four isotopic forms have been reported recently. In that work, spectral intensities were separated into the integrated intensities of different transitions by comparing the spectra of different isotopomers, and dipole moment derivatives with respect to valence displacements were calculated under the simplest approximations. For many bands it was not possible to determine the integrated intensity in this way because of overlap of several bands, and for others it was clear that the determination was too subjective. This paper first describes an attempt to improve this situation by using a more objective separation of the contributions to the intensity from different bands, by fitting the imaginary molar polarizability spectra with classical damped harmonic oscillator bands or Gaussian bands and calculating the entire area under each component band. The integrated intensities so obtained are compared with those reported previously, and a set of accepted integrated intensities for all vibrations is presented. These accepted intensities are then converted to transition moments and analyzed to obtain the dipole moment derivatives with respect to symmetry coordinates, ∂μ ∂S . The analysis uses the eigenvectors from a normal coordinate calculation that fits the reliably known fundamental wavenumbers of CH 3OH, CH 3OD, CD 3OH and CD 3OD, corrected for anharmonicity where possible, to better than ± 1.5 cm −1 on average, and that also fits the experimental near-identity of the wavenumbers and intensities of the CO stretching bands of CH 3OH and CH 3OD. These calculations were guided by literature ab initio calculations on isolated CH 3OH, but an empirical normal coordinate calculation was preferred because the experimental data show clearly that some of the vibrations are not properties of isolated molecules. For lack of other evidence, the directions of the dipole moment derivatives of the A′ modes were taken from Torii and Tasumi's recent ab initio calculation. Dipole moment derivatives with respect to internal coordinates, ∂μ ∂R , were calculated from the ∂μ ∂S . The resulting values for liquid methanol are compared with values for the isolated molecule calculated with an MP2 6-31 G ext basis set by Torii and Tasumi. For the stronger fundamentals the agreement is good except for the OH and OD stretching vibrations. This suggests that the only hydrogen vibration whose intensity is strongly affected by the hydrogen bonding is the stretching vibration. This in turn implies that it is the charge flux, not the effective charge on the hydrogen atom, that is sensitive to hydrogen bonding. The results of this and other work from this laboratory suggest that most vibrational intensities may not be strongly dependent on phase.

A Comparative Study of the Diatomic Halogen Oxides in Their Ground Electronic States

The known diatomic halogen oxides XO (X = I−F) are studied using the all-electron QCISD and QCISD(T) methods of electronic structure theory with the 6-311+G(3df) basis set. From extensive comparisons to the available ground-state spectroscopic data on XO, these methods are found to give usefully accurate values of the molecular constants, vibrationally averaged electric dipole moments, fundamental vibrational transition moments (μ01), and absolute infrared absorption intensities (S01(T)) of IO, BrO, and ClO, although certain difficulties describing FO are encountered. The all-electron, QCISD(T)-based G2 and G2(QCI) methods are used to calculate dissociation energies (D0) and adiabatic electron affinities (EA0) and ionization energies (IE0). While the G2(QCI) energies are in reasonable accord with all of the well-determined spectroscopic values, some of the G2 EA0(XO) are not because of reinforcing basis set additivity errors. Halogen group trends in the physical properties of XO (X ≠ At) are highlighted, and predictions of μ01 = −0.008 D, S01(298) = 0.4 cm-2 atm-1, and IE0 = 9.8 eV for IO and re = 1.806 Å for IO+ are made. Using G2(QCI)-like energies, we have determined the ideal gas heat of formation of IO, ΔHf°(0), to be 31.0 ± 1.0 kcal mol−1. Thermodynamic data for IO are derived in the 0−350 K range and used to compute various reaction energies of importance to atmospheric chemistry, which are briefly discussed. Also derived are the D0 = 37.9 ± 1.2 kcal mol-1 and ΔHf°(0) = −23.8 ± 1.1 kcal mol-1 of IO− and the D0 = 68.7 ± 2.2 kcal mol-1 and ΔHf°(0) = 257.0 ± 2.2 kcal mol-1 of IO+.

Determination and Use of Secondary Infrared Intensity Standards

The presentation of absorption intensities in infrared spectra is usually limited to relative intensities instead of absolute intensities. The measurement of absolute intensities can be facilitated by the use of secondary intensity standards. Such standards have been accepted by the Commission on Molecular Structure and Spectroscopy and the Physical Chemistry Division of the International Union of Pure and Applied Chemistry and were published recently. The secondary standards are based on the complex refractive index and molar absorption coefficient spectra of benzene, chlorobenzene, toluene, and dichloromethane. They have been used in this laboratory to calibrate the effective pathlength of a transmission cell and the effective number of reflections in a Circle® multiple attenuated total reflection cell. A computer program, IRYTRUE, has been developed to standardize the routine use of these intensity standards to calibrate the effective pathlength of a transmission cell. The program has been used to calibrate three transmission cells. The agreement between the calibrated values of the effective pathlength obtained from the use of different standard band groups was determined. The calibrated cell pathlength agrees with that calculated from the interference fringe pattern of the empty cell within 3% for very thin cells and within 1% for cells thicker than 100 μm. We propose that the effective pathlength evaluated in this manner be called the cell constant, and that this cell constant be used in place of the pathlength in quantitative infrared analysis. The calibration of multiple attenuated total reflection measurements in the Circle cell has been achieved in two ways: by the use of peak heights and by the use of areas. Programs PCCALC and CIRCLCAL and its associated program RSCALC are described for this purpose. The intensity standards allow one to measure absolute infrared absorption intensities of liquids with confidence to an estimated accuracy of 2–3% by either transmission or calibrated ATR methods.

Measurement and use of absolute infrared absorption intensities of neat liquids

Abstract This paper describes the methods used in this laboratory to measure absolute infrared absorption intensities of liquids and their applications in analytical and physical chemistry. The applications include the measurement of 43 bands of benzene, toluene, chlorobenzene and dichloromethane as secondary standards of infrared absorption for the International Union of Pure and Applied Chemistry, the use of these standards to calibrate transmission cells, measurement of the absolute intensity spectra of liquid benzene, the comparison of absorption intensities and transition moments in liquid and gaseous benzene, the separation of the integrated intensity of benzene into contributions from different transitions by curve fitting, and the determination of intensities and transition moments of the OH, OD, CH and CD stretching bands of CH 3 OH, CH 3 OD, CD 3 OH and CD 3 OD. The accuracy of the absolute intensities is impressively shown by the close similarity of the intensities of corresponding bands in the spectra of different isotopomers of methanol. One key to the measurement of accurate intensities is the determination of the baseline absorption.

Infrared intensities of liquids. XVII. Infrared refractive indices from 8000 to 350 cm−1, absolute integrated absorption intensities, transition moments, and dipole moment derivatives of methan-d3-ol and methanol-d4 at 25 °C

This paper reports absolute infrared absorption intensities of liquids methan-d3-ol (CD3OH) and methanol-d4 (CD3OD) at 25 °C between 8000 and 350 cm−1. Measurements were made by multiple attenuated total reflection spectroscopy with the CIRCLE cell, and by transmission spectroscopy with transmission cells fitted with calcium fluoride windows. In both cases, the spectra were converted to infrared real and imaginary refractive index spectra. The refractive indices obtained by these two methods agreed excellently and were combined to yield an imaginary refractive index spectrum k(ν̃) between 7244 and 350 cm−1 for CD3OH and between 5585 and 350 cm−1 for CD3OD. The imaginary refractive index spectrum was arbitrarily set to zero from 8000 to 7244 cm−1 (CD3OH) or 5585 cm−1 (CD3OD), where k is always less than 4×10−6, in order that the real refractive index can be calculated below 8000 cm−1 by Kramers–Krönig transformation. The results are reported as graphs and tables of the refractive indices between 8000 and 350 cm−1, from which all other infrared properties of the two liquids can be calculated. The estimated accuracy, not precision, of the imaginary refractive index is ±3%, except for ±10%, where k is less than 4×10−5. The estimated accuracy of the real refractive index is better than ±0.5%. In order to obtain molecular information from the measurements, the spectra of the imaginary polarizability multiplied by wave number ν̃αm″ were calculated under the assumption of the Lorentz local field. The area under these ν̃αm″ spectra was separated into the integrated intensities of different vibrations. The magnitudes of the transition moments were calculated from the integrated intensities, and the double harmonic approximation was used to calculate the magnitudes of the dipole moment derivatives of the liquid-state molecules with respect to the normal coordinates. Dipole moment derivatives with respect to internal coordinates were calculated under the simplest approximations, the validity of which is demonstrated by the experimental data in many cases. The consistency of the dipole moment derivatives with respect to internal coordinates obtained for different isotopomers is shown through their relative rotational corrections. Results are presented for the O–H, O–D, C–H, and C–D stretches; the C–O–H in-plane bending; and the D–C–O–H and D–C–O–D torsion vibrations.

Comparison of infrared absorption intensities of benzene in the liquid and gas phases

This paper presents a comparison of the absolute infrared absorption intensities in the liquid and gas phases for the four infrared active fundamentals of benzene. In Herzberg’s notation these are ν12 (∼3070 cm−1), and ν4 (∼675 cm−1). Published data are used, including the recently published spectra of liquid benzene that have been accepted by the International Union of Pure and Applied Chemistry as secondary intensity standards. The present results agree qualitatively with the conclusions drawn in 1970 that the intensity Aj of ν12 is much smaller for the liquid than for the gas, and those of ν13, ν14, and ν4 are all larger for the liquid. The inclusion of measurements made since 1970 should make the quantitative results reported here the most reliable. However, two quite different values have been reported in the 1980’s for the intensity of ν14 in the gas phase, and both are considered. The comparison for ν14 is also complicated by the existence of weak bands in the spectrum of the liquid that are not observed in that of the gas. It is noted in this work that the traditional comparison, of the areas under the molar absorption coefficient spectra, Aj, for the gas and liquid through the Polo–Wilson equation, has the drawback that the ratio expected if the dipole moment derivative is unchanged is different for each band as well as for each liquid. A much more convenient ratio, that equals unity for all bands of all liquids under the traditional assumptions, is proposed through the imaginary molar polarizability spectrum of the liquid. The magnitudes of the transition moments and the dipole moment derivatives with respect to the normal coordinates under the double harmonic approximation are calculated from the measured intensities for the gas and liquid phases. It is found that the dipole moment derivative of ν12 is 24% smaller in the liquid than in the gas and that of ν13 is 18% larger. The dipole moment derivative of ν4 is unchanged by condensation. The change in the dipole moment derivative of ν14 is not clear, because of the uncertainty in the gas phase intensity and because of the uncertain origin of the intensity of the additional bands in the liquid.

Vibrational intensities of lattice modes and the distributed dipole model: Crystalline acetylene

The vibrational intensities of crystalline acetylene are calculated using models of increasing complexity for the molecular charge distributions and polarizabilities. It is found that it is necessary to include high order molecular multipole moments, represented by a distributed dipole model, to account for the absolute infrared absorption intensities of the two crystal phases. High order molecular polarizabilities, represented by distributed dipolar polarizabilities, have an important effect on the calculated absolute intensities and susceptibilities, although the relative intensity patterns are not too much model sensitive.

Absolute Intensity Measurement of the ν4 Band of Fluoroform by the Use of a Tunable Diode-laser Source

Abstract The absolute infrared absorption intensities have been measured for the P35(35) and P36(36) lines, which belong to the ν4 fundamental of fluoroform, by the use of a tunable diode-laser source. The observed intensity, Γ, was 5360+220 cm2 mol−1. This intensity value was compared with that observed by the conventional method.

Enthalpy difference of rotational isomers in liquid butane and pentane from infrared spectra

In the course of a study of the absolute infrared absorption intensities of the fundamentals of liquid normal alkanes at different temperatures, the authors derived the enthalpy difference between the conformational isomers of liquid butane and pentane.(AIP)

Absolute Intensity Measurement of Infrared Absorption by the Use of CO2 Laser Source

Abstract The absolute infrared absorption intensities have been measured for the QR0(0) line (ν3 fundamental) of methyl fluoride and the RR8(9) line (ν6 fundamental) of methyl iodide by the use of CO2 laser lines of P(16) and P(8), respectively. The observed intensities, Γ, were: 12500±4300 cm2·mol−1 for the ν3 band of methyl fluoride and 1040±40 cm2·mol−1 for the ν6 band of methyl iodide.

The Analysis of the Molecular Vibration by the Molecular Orbital Method. I. Application to HCN

Abstract The approximate self-consistent molecular orbital method has been applied to the analysis of the vibrational spectrum of HCN. The frequencies of the fundamental, overtone, and combination bands in the region from about 700 to 12000 cm−1 have been calculated, and their absolute infrared absorption intensities have been estimated, the variational method being used in this calculation. Also, the frequencies and absorption intensities of hot bands, referring to the bending vibration, have been calculated. In comparison with the experimental results, the calculated frequencies were not always appropriate. However, it was found that these results are effective in the analysis of the molecular vibration if the calculated frequencies are discussed along with their infrared absorption intensities.

Studies in vibrational absorption intensities: Vibronic effects in hexafluorobenzene and benzene

The absolute infrared absorption intensities of the low frequency fundamentals of hexafluorobenzene were measured—both by direct integration of extinction coefficients and by the P, R band maximum technique. An analysis of results in terms of the simple bond moment hypothesis and of an electron rehybridization moment accompanying the out-of-plane substituent deformation has led to value of 0.3 D/radian for the latter. This is equal to the value for benzene. The insensitivity of both the C 6F 6 and the C 6H 6 results on the force fields, and the e 1 u first-order Coriolis coupling constants in particular, is sufficiently low for the equality of the results to be significant.

Absolute infrared intensities in crystalline acetylene and ethane

Absolute infrared absorption intensities have been measured for three bands of crystalline acetylene and two bands (the hydrogen stretching motions) of crystalline ethane. The changes of intensity observed on condensation of the gases cannot be explained as a “dielectric effect”. In acetylene the large frequency shift and intensification of the hydrogen stretching mode are consistent with characterization of the intermolecular forces as hydrogen bonding to the acetylenic π-clouds.

Infrared Intensities of the Fundamental Vibrations of GeH4 and GeD4

The absolute infrared absorption intensities of the fundamental vibrations of GeH4 and GeD4 have been measured using nitrogen pressures of 900 psi to broaden the individual rotational lines. Alternative sets of the derivatives of the dipole moment with respect to designated stretching and bending symmetry coordinates are obtained. A least-squares analysis of the isotopic data indicates that the positive ratio of the two dipole-moment derivatives is the preferred solution.

Atomic Polarization of Sulfur Hexafluoride and of Carbon Tetrafluoride

A microwave cavity technique, operating at a frequency of 800 Mc/sec, has been used to measure the dielectric constants of SF6 and CF4 between 295° and 630°K. Over this temperature range the molar polarization has been shown to be independent of temperature within an accuracy of a few tenths of a percent. The atomic polarization is calculated and compared with previous measurements of absolute infrared absorption intensities for the two gases. The use of precise dielectric constant measurements as a check on the accuracy of infrared intensity measurements is discussed.