Quantum and classical IR spectra of (HCOOH)2, (DCOOH)2 and (DCOOD)2 using ab initio potential energy and dipole moment surfaces (original) (raw)
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The infrared spectrum of the H2–HCO+ complex
The Journal of Chemical Physics, 1995
A combined experimental and theoretical study of the structural properties of the H 2 -HCO ϩ ion-neutral complex has been undertaken. Infrared vibrational predissociation spectra of mass selected H 2 -HCO ϩ complexes in the 2500-4200 cm Ϫ1 range display several vibrational bands, the most intense arising from excitation of the C-H and H 2 stretch vibrations. The latter exhibits resolved rotational structure, being composed of ⌺-⌺ and ⌸-⌸ subbands as expected for a parallel transition of complex with a T-shaped minimum energy geometry. The determined ground state molecular constants are in good agreement with ones obtained by ab initio calculations conducted at the QCISD͑T͒/6 -311G(2d f ,2pd) level. The complex is composed of largely undistorted H 2 and HCO ϩ subunits, has a T-shaped minimum energy geometry with an H 2 •••HCO ϩ intermolecular bondlength of approximately 1.75 Å. Broadening of the higher J lines in the P and R branches of the ⌸-⌸ subband is proposed to be due to asymmetry type doubling, the magnitude of which is consistent with the calculated barrier to H 2 internal rotation. The lower J lines in the ⌺-⌺ and ⌸-⌸ subbands have widths of 0.06 cm Ϫ1 , around three times larger than the laser bandwidth, corresponding to a decay time of Ϸ90 ps for the upper level. The absence of discernible rotational structure in the 2 band suggests that it has predissociation lifetime of less than 1 ps.
Ab initio computations on H2S: LCAOSCF wave functions without d orbitals
Theoretica Chimica Acta, 1970
Ab initio MO-SCF wave functions are derived for HaS for different bond angles by using a double-zeta type set of gaussian s and p orbitals. The predicted equilibrium bond angle is 95.5 ~ The computed value of the total electronic energy is expected to be near the Hartree-Fock limit for the molecule. The predicted value of the dipole moment does not show significant improvement with respect to similar computations not including polarisation functions.
Model infrared spectra for non-rotating H 2 O are calculated at 0 K, based on exact quantum, standard classical and semiclassical calculations. An accurate potential energy surface is used along with a realistic dipole function. An analysis of the classical and quantum spectrum in the harmonic approximation is presented at 0 K. This clearly reveals that the magnitude of the classical intensities is essentially arbitrary, depending on the total energy. Thus, the intensity of classical harmonic spectrum disagrees with the corresponding quantum one. A very simple correction to the classical spectrum is suggested that largely restores agreement with the harmonic quantum spectrum. A second, more general classical correction is also suggested, which, however, requires knowledge of the normal modes.
The infrared spectrum of He–HCO+
The Journal of Chemical Physics, 1995
A combined experimental and theoretical study of the structural properties of the H 2 -HCO ϩ ion-neutral complex has been undertaken. Infrared vibrational predissociation spectra of mass selected H 2 -HCO ϩ complexes in the 2500-4200 cm Ϫ1 range display several vibrational bands, the most intense arising from excitation of the C-H and H 2 stretch vibrations. The latter exhibits resolved rotational structure, being composed of ⌺-⌺ and ⌸-⌸ subbands as expected for a parallel transition of complex with a T-shaped minimum energy geometry. The determined ground state molecular constants are in good agreement with ones obtained by ab initio calculations conducted at the QCISD͑T͒/6 -311G(2d f ,2pd) level. The complex is composed of largely undistorted H 2 and HCO ϩ subunits, has a T-shaped minimum energy geometry with an H 2 •••HCO ϩ intermolecular bondlength of approximately 1.75 Å. Broadening of the higher J lines in the P and R branches of the ⌸-⌸ subband is proposed to be due to asymmetry type doubling, the magnitude of which is consistent with the calculated barrier to H 2 internal rotation. The lower J lines in the ⌺-⌺ and ⌸-⌸ subbands have widths of 0.06 cm Ϫ1 , around three times larger than the laser bandwidth, corresponding to a decay time of Ϸ90 ps for the upper level. The absence of discernible rotational structure in the 2 band suggests that it has predissociation lifetime of less than 1 ps.
High resolution analysis of the ν 7 and ν 9 bands of DCOOH
Journal of Molecular Spectroscopy, 2003
The 7 1 and 9 1 vibrational states of deuterated species of formic acid molecule DCOOH have been recorded by a FTIR spectrometer in the region 450-820 cm À1 at a resolution of 0:00125 cm À1 and a millimeter wave spectrometer. In the analysis microwave transitions from literature were used in addition to 14 835 assigned IR and 114 millimeter wave lines in the 7 1 and 9 1 vibrational states. The analysis resulted in band origins, rotational, centrifugal distortion, and eight interaction parameters of the Coriolis coupled 7 1 and 9 1 vibrational states. RMS deviation of the fit was 0:000176 cm À1 for the IR data and the maximum values of J and K a quantum numbers in the fit were 64, 28 and 64, 30 for 7 1 and 9 1 states, respectively.
The Journal of Chemical Physics, 2001
The gas phase IR spectrum of the O¯H¯O fragment of H 5 O 2 ϩ and its deuterated analogue are calculated using ab initio classical molecular dynamics based on a MP2 potential energy surface. The assignment of the bands is made in terms of the quantum four-dimensional model calculations of anharmonic frequencies and intensities. Comparing low and high kinetic temperature simulations the importance of anharmonicities of the potential energy surface for understanding the vibrational band structure is highlighted. It is shown that any reasonable simulation of IR spectra of systems with very strong hydrogen bonds has to account for the dipole moment function beyond the linear approximation.
Chemical Physics, 1996
Ab initio calculations using the 6-311G(d, p) basis set at the QCISD and CCSD(T) levels were undertaken on the ground and lowest excited electronic states of the isoelectronic carbon-chain free radicals, HC4H 2, HC3NH and HC30. The equilibrium geometry for each molecule is a C s structure with four electrons in a" orbitals, and the lowest excited states are characterized by configurations with three, five, and six d' electrons. The effective vibrational potential energy surfaces of these states along the two lowest frequency in-plane bending coordinates were investigated in order to assess the trend in large-amplitude motions and isomerization across different electronic configurations in a series of isoelectronic molecules. The (4-d') ground state potential surfaces of HC3NH and HC30 are subject to the Renner-Teller effect, and intersect the (5-d') excited state surfaces at linear 211 states at 2300 cm-l and 8400 cm-l, respectively, above the energy of the equilibrium geometries. The (3-a") states of HC30 and HC3NH lie 27000 cm -l and 24000 cm-l above the (4-d') equilibrium states, respectively, and form Renner-Teller pairs with the (6-d') states. Although two minima are found for HC30 and HC3NH on the (3-d') surface, none of these minima is stable with respect to planarity. The (5-d') surface of HC4H 2 has a single minimum at a C2v structure, which is unstable with respect to the ground state. The (3-d') surface has two minima, both of which are stable. Equilibrium structures, excitation energies, and potential surface properties for these structures are reported.
An ab initio calculation of the vibrational energies and transition moments of HSOH
Journal of Molecular Spectroscopy, 2009
We report new ab initio potential energy and dipole moment surfaces for the electronic ground state of HSOH, calculated by the CCSD(T) method (coupled cluster theory with single and double substitutions and a perturbative treatment of connected triple excitations) with augmented correlation-consistent basis sets up to quadruple-zeta quality, aug-cc-pV(Q+d)Z. The energy range covered extends up to 20 000 cm À1 above equilibrium. Parameterized analytical functions have been fitted through the ab initio points. Based on the analytical potential energy and dipole moment surfaces obtained, vibrational term values and transition moments have been calculated by means of the variational program TROVE. The theoretical term values for the fundamental levels m SH (SH-stretch) and m OH (OH-stretch), the intensity ratio of the corresponding fundamental bands, and the torsional splitting in the vibrational ground state are in good agreement with experiment. This is evidence for the high quality of the potential energy surface. The theoretical results underline the importance of vibrational averaging, and they allow us to explain extensive perturbations recently found experimentally in the SH-stretch fundamental band of HSOH.
Chemical Physics, 2008
We report the calculation of a six-dimensional CCSD(T)/aug-cc-pVQZ potential energy surface for the electronic ground state of NH þ 3 together with the corresponding CCSD(T)/aug-cc-pVTZ dipole moment and polarizability surface of 14 NH þ 3 . These electronic properties have been computed on a large grid of molecular geometries. A number of newly calculated band centers are presented along with the associated electric-dipole transition moments. We further report the first calculation of vibrational matrix elements of the polarizability tensor components for 14 NH þ 3 ; these matrix elements determine the intensities of Raman transitions. In addition, the rovibrational absorption spectra of the m 2 , m 3 , m 4 , 2m 2 À m 2 , and m 2 þ m 3 À m 2 bands have been simulated.
The Journal of Physical Chemistry, 1994
Density functional theory at the local and gradient-corrected (BeckePerdew and Becke-Lee, Yang, Parr) levels has been used to predict the geometries, frequencies, and infrared intensities for H20, HOO, CH4, and C2H4. Large basis sets have been emplyed in this study. An extensive set of calculations was performed for H2O and HOO, whereas for CH4 and C2H4 calculations were only done at the highest level. The calculated infrared intensities at the BP/TZVPD level are ZI = 1.8,12 = 68.9, and = 49.9 km/mol for H2O; Zl = 11.8, Z2 = 33.8, and 13 = 16.8 km/mol for HOO; Z(t1,-str) = 60.3 and Z(tl,-bend) = 37.0 km/mol for CH4; and Z(v7) = 96.7, Z(v9) = 19.3, Z(v10) = 0.2, I(vl1) = 17.6, and Z(v12) = 9.3 km/mol for C2H4.