M. Carvajal

Vibrational energies for NH3 based on high level ab initio potential energy surfaces

Ab initio coupled cluster calculations with single and double substitutions and a perturbative treatment of connected triple substitutions [CCSD(T)] have been carried out to generate six-dimensional (6D) potential energy surfaces (PES) and dipole moment surfaces (DMS) for the electronic ground state of ammonia. Full 6D-PES and 6D-DMS (14400 points) were computed with the augmented correlation-consistent triple-zeta basis (aug-cc-pVTZ). For a selected number of points (420 in C3v symmetry and 1260 in lower symmetry), more accurate energies (CBS+) were obtained by extrapolating the CCSD(T) results for the aug-cc-pVXZ (X=T,Q,5) basis sets to the complete basis set limit and adding corrections for core-valence correlation and relativistic effects. Two procedures were investigated to enhance the quality of the 6D-PES from CCSD(T)/aug-cc-pVTZ by including the CBS+ data points. The resulting 6D-PES were represented by analytical functions involving Morse variables for the stretches, symmetry-adapted bending coor...

Rotational spectrum of 13C2-methyl formate (HCOO13CH3) and detection of the two 13C-methyl formate in Orion

Context. Laboratory measurements and analysis of the microwave and millimeter-wave spectra of potential interstellar molecules are a prerequisite for their subsequent identification by radioastronomical techniques. The spectral analysis provides spectroscopic parameters that are used in the assignment procedure of the laboratory spectra, and that also predict the frequencies of transitions not measured in the laboratory with a high degree of precision. Aims. An experimental laboratory study and its theoretical analysis is presented for 13 C2-methyl formate (HCOO 13 CH3) allowing a search for this isotopologue in the Orion molecular cloud. The 13 C1-methyl formate (H 13 COOCH3) molecule was also searched for in this interstellar cloud, using previously published spectroscopic data. Methods. The experimental spectra of 13 C2-methyl formate were recorded in the microwave and sub-mm energy ranges (4–20 GHz, 8–80 GHz, 150–700 GHz). The spectra were analyzed using the Rho-Axis Method (RAM), which takes the CH3 internal rotation and the coupling between internal rotation and global rotation into account. Results. Twenty-seven spectroscopic constants of 13 C2-methyl formate have been obtained from a fit of 936 transitions of the ground torsional state with a standard (unitless) deviation of 1.08. A prediction of line positions and intensities is also produced. This prediction allowed us to identify 230 13 C2-methyl formate lines in the Orion interstellar molecular cloud. We refitted all previously published ground state transitions of the 13 C1-methyl formate molecule in order to provide a prediction of its ground state spectrum. 234 lines of 13 C1-methyl formate were detected in the Orion interstellar cloud using that prediction.

Rotation–vibration motion of pyramidal XY3 molecules described in the Eckart frame: Theory and application to NH3

We present a new model for the rotation-vibration motion of pyramidal XY3 molecules, based on the Hougen–Bunker–Johns approach. Inversion is treated as a large-amplitude motion, while the small-amplitude vibrations are described by linearized stretching and bending coordinates. The rotation–vibration Schrödinger equation is solved variationally. We report three applications of the model to 14NH3 using an analytic potential function derived from high-level ab initio calculations. These applications address the J = 0 vibrational energies up to 6100 cm, the J≤2 energies for the vibrational ground state and the ν2, ν4, and 2ν2 excited vibrational states, and the J≤7 energies for the vibrational state. We demonstrate that also for four-atomic molecules, theoretical calculations of rotation–vibration energies can be helpful in the interpretation and assignment of experimental, high-resolution rotation–vibration spectra. Our approach incorporates an optimum inherent separation of different types of nuclear motion and thus remains applicable for rotation–vibration states with higher J values where alternative variational treatments are no longer feasible.

Potential parameters of PH3 obtained by simultaneous fitting of ab initio data and experimental vibrational band origins

Abstract We report here the experimental observation, by photoacoustic spectroscopy, of transitions to the (600 A 1 /E) local mode states of PH 3 . The vibrational energies for these two states are used, together with all other experimentally derived vibrational energies for PH 3 , as input for a least-squares refinement of the potential energy surface for the electronic ground state. We propose a procedure for simultaneously fitting the experimental data and ab initio values for the potential energy. By employing this procedure, we circumvent the problem of unrealistic behaviour of the fitted potential energy surface caused by the shortage of experimental data.

Rotational Spectrum and Tentative Detection of DCOOCH3-Methyl Formate in Orion

New centimeter-wave (7-80 GHz) and submillimeter-wave (580-661 GHz) spectra of a deuterated species of methyl formate (DCOOCH3) have been measured. Transitions with a maximum value of J = 64 and K = 36 have been assigned and fitted together with previous measurements. The internal rotation of this compound was treated using the so-called rho axis method. A total of 1703 transitions were fitted using this method. Only 24 parameters were employed in the final fit, which has an rms deviation of 94.2 kHz. The dipole moment and the nuclear quadrupole coupling constants of the deuterated specie have also been obtained. This new study has permitted a tentative detection of DCOOCH3 in Orion with the IRAM 30 m telescope based on the observation of more than 100 spectral features with low blending effects among the 400 lines expected in the observed frequency domain (for which over 300 are heavily blended with other species). These 100 transitions are above noise and confusion limited without heavy blending and cannot be assigned to any other species. Moreover, none of the strongest unblended transitions is missing. The derived source-averaged total column density for DCOOCH3 is 7.8 × 1014 cm–2 and the DCOOCH3/HCOOCH3 column density ratio varies between 0.02 and 0.06 in the different cloud components of Orion. This value is consistent with the deuteration enhancement found for other species in this cloud.

Dipole moment and rovibrational intensities in the electronic ground state of NH3: Bridging the gap between ab initio theory and spectroscopic experiment

We report theoretical values for the transition moments of an extensive set of vibrational bands in the electronic ground state of (14)NH(3). For selected bands, we have further made detailed simulations of the rotational structure. The calculations are carried out by means of recently developed computational procedures for describing the nuclear motion and are based on a high-level ab initio potential energy surface, and high-level dipole moment surfaces, for the electronic ground state of NH(3). The reported theoretical intensity values are compared to, and found to agree very well with, corresponding experimental results. It is believed that the computational method, in conjunction with high-quality ab initio potential energy and dipole moment surfaces, can simulate rotation-vibration spectra of XY(3) pyramidal molecules prior to observation with sufficient accuracy to facilitate the observation of these spectra. By degrading the accuracy of selected elements of the calculations, we have also investigated the influence of customary approximations on the computed intensity values.

Microwave and submillimeter spectroscopy and first ISM detection of 18O-methyl formate

Context. Astronomical survey of interstellar molecular clouds needs a previous analysis of the spectra in the microwave and sub-mm energy range to be able to identify them. We obtained very accurate spectroscopic constants in a comprehensive laboratory analysis of rotational spectra. These constants can be used to predict transition frequencies that were not measured in the laboratory very precisely. Aims. We present an experimental study and a theoretical analysis of two 18 O-methyl formate isotopologues, which were subsequently detected for the first time in Orion KL. Methods. The experimental spectra of both methyl formate isotopologues recorded in the microwave and sub-mm range from 1 to 660 GHz. Both spectra were analysed by using the rho-axis method (RAM) which takes into account the CH3 internal rotation. Results. We obtained spectroscopic constants of both 18 O- methyl formate with high accuracy. Thousands of transitions were assigned and others predicted, which allowed us to detect both species in the IRAM 30 m line survey of Orion KL.

Rotation-vibration motion of pyramidal XY₃ molecules described in the Eckart frame: The calculation of intensities with application to NH₃

Abstract We present a theoretical model, with accompanying computer program, for simulating rotation–vibration absorption spectra of XY 3 pyramidal molecules in isolated electronic states. The theoretical approach is based on a recent computational scheme for solving the rotation–vibration Schrodinger equation of such molecules variationally [S. N. Yurchenko, M. Carvajal, P. Jensen, H. Lin, J. Zheng, and W. Thiel, Mol. Phys. , 2005, 103 , 359], and it makes use of dipole moment surfaces calculated ab initio . We apply the theory to 14 NH 3 and demonstrate that the theoretical results show good agreement with experimental findings.

Vibrational energies of PH3 calculated variationally at the complete basis set limit

The potential energy surface for the electronic ground state of PH(3) was calculated at the CCSD(T) level using aug-cc-pV(Q+d)Z and aug-cc-pVQZ basis sets for P and H, respectively, with scalar relativistic corrections included. A parametrized function was fitted through these ab initio points, and one parameter of this function was empirically adjusted. This analytical PES was employed in variational calculations of vibrational energies with the newly developed program TROVE. The convergence of the calculated vibrational energies with increasing vibrational basis set size was improved by means of an extrapolation scheme analogous to the complete basis set limit schemes used in ab initio electronic structure calculations. The resulting theoretical energy values are in excellent agreement with the available experimentally derived values.

Global assignment and extension of millimeter- and submillimeter-wave spectral database of 13C1-methyl formate (H13COOCH3) in the ground and first excited states

A compilation of the available spectroscopic millimeter- and submillimeter-wave data of the ground and first excited states of 13C1-methyl formate (H13COOCH3) has been carried out. The exhaustive analysis of the available transition lines of H13COOCH3 has led to the assignment of 7457 spectral lines by means of a global fit of 45 parameters, using the Rho-Axis Method and the BELGI-Cs code, with a resulting unitless standard deviation of 0.57. Over 1600 lines are included for the first time in the fit. In addition, the line strengths of spectral lines are also calculated using the most recent experimental measurement of the electric dipole moment. In conclusion, the present study represents a notable improvement with respect to previous H13COOCH3 spectral analyses. Therefore, the better accuracy of the present analysis may help the future identification of new H13COOCH3 lines in the interstellar and circumstellar media, and may contribute to decrease some of the spectral confusion due to these species in astronomical surveys.