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D J DeFrees

Publications and source records attributed to D J DeFrees.

30 records · Page 2Linked to original sources
PubMed1987

Collisional excitation of interstellar cyclopropenylidene.

Theoretical rotational excitation rates were computed for C3H2 in collisions with He atoms at temperatures from 30 to 120 K. The intermolecular forces were obtained from accurate self-consistent field and perturbation theory calculations, and collision dynamics were treated within the infinite-order sudden approximation. The accuracy of the latter was examined by comparing with the more exact coupled states approximation.

Astronomical Phenomena↗
PubMed1986

A priori predictions of the rotational constants for protonated formaldehyde and protonated methanol.

Protonated formaldehyde and protonated methanol are candidate interstellar molecules and models for classes of protonated oxygen compounds. Ab initio molecular orbital theory has been used to compute rotational constants to guide spectroscopic searches both in the laboratory and in space. The ab initio results are empirically corrected to account for systematic deficiencies in the theory and zero-point vibrational effects; they are expected to be accurate to approximately +/-2%. For H2COH+ the resultant constants are (in GHz) A = 194.3, B = 34.28, and C = 29.14; for H3COH2+ A = 103.7, B = 21.18, and C = 20.30.

Astronomical Phenomena↗
PubMed1986

Is N-protonated hydrogen isocyanide, H2NC+, an observable interstellar species?

Ab initio molecular orbital theory is used to examine the singlet and triplet potential energy surfaces for the CH2N+ system. The results confirm those of earlier studies which suggested that the singlet H2NC+ isomer could be formed via the corresponding triplet isomer. Also, it is shown that the reaction HCN+ + H2 might lead to this metastable isomer without invoking the triplet species. The best test of the hypothesis that this molecule can be formed by gas phase, ion molecule reactions and may be an important precursor in the interstellar synthesis of HCN and HNC is to search for it in space. To this end, theoretical predictions are made of its rotational frequencies and its vibrational frequencies and intensities to serve as a guide to laboratory spectroscopists and radioastronomers.

Astronomical Phenomena↗
PubMed1986

Ab initio molecular orbital studies of low-energy, metastable isomers of the ubiquitous cyclopropenylidene.

The discovery of cyclopropenylidene in space suggests that other C3H2 isomers may be present and a tentative detection of one such isomer, propargylene (HCCCH), has been reported. Ab initio molecular orbital theory has been used to characterize five low-lying, metastable isomers of cyclopropenylidene. Four are found to be stable minima and their vibrational frequencies are reported, although the stable form of triplet propargylene is not firmly established. Extended calculations including the electron correlation energy, show that the lowest in energy is singlet propadienylidene, followed by propargylene; the singlet and triplet of the latter are too close in energy to allow an assignment of the ground state; triplet propadienylidene is at a significantly higher energy. Rotational frequencies computed to an expected accuracy of +/- 1%-2% do not confirm the tentative detection of propargylene in space, although the discrepancy between theory and the observation is not so great as to unequivocally rule out this possibility.

Astronomy↗
PubMed1986

A reanalysis of the HCO+/HOC+ abundance ratio in dense interstellar clouds.

New theoretical and experimental results have prompted a reinvestigation of the HCO+/HOC+ abundance ratio in dense interstellar clouds. These results pertain principally but not exclusively to the reaction between HOC+ and H2, which was previously calculated by DeFrees, McLean, and Herbst to possess a large activation energy barrier. New calculations, reported here, indicate that this activation energy barrier is quite small and may well be zero. In addition, experimental results at higher energy and temperature indicate strongly that the reaction proceeds efficiently at interstellar temperatures. If HOC+ does indeed react efficiently with H2 in interstellar clouds, the calculated HCO+/HOC+ abundance ratio rises to substantially greater value under standard dense cloud conditions than in deduced via the tentative observation of HOC+ in Sgr B2.

Astronomical Phenomena↗
PubMed1986

Ab initio determination of the proton affinities of small neutral and anionic molecules.

The proton affinity of a molecule in the gas phase is a fundamental measure of its basicity and is the factor controlling the course of many ion-molecule reactions. In this article, ab initio molecular orbital theory at the MP4/6-311 ++ G(3df, 3pd) level of theory is demonstrated to predict proton affinities (PA's) for small neutral and anionic bases to within 2 kcal mol-1. Furthermore, the errors are random, indicating that there are likely no systematic errors in either the experimental or theoretical PA's. Also, this level of theory is used to calibrate less sophisticated theoretical models which are suitable for larger molecules; the MP4/6-311 ++ G(2d, 2p) and MP2/6-311 ++ G(d, p) theoretical models should be particularly useful. A procedure for predicting the vibrational frequencies for anion is proposed and applied to CH3-, NH2-, OH-, and CN-.

Anions↗
PubMed1985

Theoretical investigation of the interstellar CH3NC/CH3CN ratio.

Calculations have been performed to determine the abundance ratio of the metastable isomer CH3NC to the stable isomer CH3CN in dense interstellar clouds. According to gas phase, ion-molecule treatments, these molecules are both synthesized via protonated ion precursors. We have calculated the ratio of the formation rates of the protonated precursor ions-- CH3NCH+ and CH3CNH+ --synthesized via the radiative association reaction between CH3+ and HCN, which is thought to the dominant formation process of the two isomeric ions. Our calculations, which involve both ab initio quantum chemistry and equilibrium determinations, lead to a predicted CH3NCH+/CH3CNH+ formation rate ratio between 0.1 and 0.4. If this ratio is maintained in the neutral species formed from the precursor ions, theory predicts a sizable abundance for methyl isocyanide (CH3NC) and lends credence to its tentative observation.

Acetonitriles↗
PubMed1985

Molecular orbital predictions of the vibrational frequencies of some molecular ions.

Recent spectroscopic advances have led to the first determinations of infrared vibration-rotation bands of polyatomic molecular ions. These initial detections were guided by ab initio predictions of the vibrational frequencies. The calculations reported here predict the vibrational frequencies of additional ions which are candidates for laboratory analysis. Vibrational frequencies of neutral molecules computed at three levels of theory, HF/3-21G, HF/6-31G*, and MP2/6-31G*, were compared with experiment and the effect of scaling was investigated to determine how accurately vibrational frequencies could be predicted. For 92% of the frequencies examined, uniformly scaled HF/6-31G* vibrational frequencies were within 100 cm-1 of experiment with a mean absolute error of 49 cm-1. This relatively simple theory thus seems suitable for predicting vibrational frequencies to guide laboratory spectroscopic searches for ions in the infrared. Hence, the frequencies of 30 molecular ions, many with astrochemical significance,were computed. They are CH2+, CH3+, CH5+, NH2+, NH4+, H3O+, H2F+, SiH2+, PH4+, H3S+, H2Cl+, C2H+, classical C2H3+, nonclassical C2H3+, nonclassical C2H5+, HCNH+, H2CNH2+, H3CNH3+, HCO+, HOC+, H2CO+, H2COH+, H3COH2+, H3CFH+, HN2+, HO2+, C3H+, HOCO+, HCS+, and HSiO+.

Astronomical Phenomena↗
PubMed1984

The quantum mechanical calculation of rotational spectra. A comparison of methods for C2H2, HCN, HNC, HCO+, N2H+, CO and N2. Predictions for HCNH+, COH+, HBO, HBNH, and HBF+.

Rotational frequencies determined with ab initio molecular orbital theory can play an important role in guiding spectroscopic searches for new molecules and in corroborating the assignment of unidentified lines, from the laboratory and from space. In a systematic study of 22 levels of molecular orbital theory, CISD/6-311G** gave rotational frequencies to an accuracy of +/- 0.4 GHz when an empirical correction is applied to the results for C2H2,HCN, HNC, HCO+, N2H+, CO, and N2. Larger errors can be expected when there are large vibrational effects on the rotational constants, as exemplified by COH+. Predicted J = 0--> 1 rotational frequencies using these methods are 73.9 +/- 0.4 GHz for HCNH+, 78.6 +/- 0.4 GHz for HBO, 65.8 +/- 0.4 GHz for HBNH, and 72.1 +/- 0.4 GHz for HBF+.

Chemical Phenomena↗
PubMed1984

Calculations concerning the HCO+/HOC+ abundance ratio in dense interstellar clouds.

Calculations have been performed to determine the rate coefficients of several reactions involved in both the formation and depletion of interstellar HCO+ and HOC+. The abundance of HOC+ deduced from these calculations is consistent with the tentative identification of HOC+ in Sgr B2 by Woods et al. The large HCO+/HOC+ abundance ratio observed by Woods et al. is due at least in part to a more rapid formation rate for HCO+ and probably due as well as to a more rapid depletion rate for HOC+.

Aldehydes↗