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Infrared Quantum-Mechanical Calculations
- Keeping Mars warm with new super greenhouse gases

Gases considered for the special purpose of maintaining warmth on Mars are shown in Table 1. To determine the structure and vibrational spectra of each gas, all calculations were performed by using the GAUSSIAN 98 suite of programs (9). All geometries were optimized fully with no constraints to better then 0.1 pm (for bond lengths) and 0.1° (for angles). The calculations were done with two kinds of basis sets: (i) the medium size double zeta-split valence basis set, 6-31G(d); and (ii) the large 6-311 ++G(3df,3pd) basis set. Optimizations were performed with the Becke nonlocal three-parameter exchange (10) and correlation functional was performed with the Lee-Yang-Parr method (B3LYP) with these basis sets (11). In all cases, harmonic vibrational frequency calculations were done with the optimized geometries. Structures giving rise to imaginary vibrational frequencies were rejected, because these structures are not global minimum structures. To determine the reliability of the various methods for predicting the vibrational frequencies for the super greenhouse gases, 10 other gases were selected as calibrants, because the fundamental vibrational modes are experimentally well characterized. These gases are CO2, CFCl3, CF2Cl2, CF2CFOCF3, CH3OCH3, CHF2OCF3, CHF2OCHF2, CH3OCF3, SF6, and NF3. Usually, vibrational frequencies calculated with lower levels of theory are found to overestimate experimental frequencies by 5-10%, so a popular approach has been to use scale factors (12). However, in the present work no scaling factors were applied. A comparison of observed and calculated frequencies for our calibration set of gases showed that calculations performed at the B3LYP/6-311 + +G(3df,3pd) level of theory produced frequencies within an rms error of 3% of experimental vibrational frequencies. Recent studies have found that the B3LYP method offers the best performance in computing intensities (13). As a further test of the reliability of the calculations, the infrared spectra from the ab initio frequencies were simulated and compared with the experimental spectra. We found that the simulated spectra showed reasonable agreement with the experimental spectra of each calibrant gas.

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