Predicted vibrational spectra from anharmonic potential functions

Thesis/Dissertation · Wed Jan 01 00:00:00 EST 1986

OSTI ID:5002247

Dunn, K M

The dissertation develops a procedure for predicting vibrational spectra of polyatomic molecules from a combination of theoretical and experimental information. Ab initio quantum chemical calculations provide anharmonic force constants including cubics and diagonal quartics. A variational procedure analogous to configuration interaction is then used to compute eigenvalues of the pure vibrational Hamiltonian. The diagonal quadratic force constants are then adjusted until the calculated fundamental frequencies agree with experiment. The resulting theoretical-experimental force field may then be used to predict the energies of vibrationally excited states. The method is applied to three molecules: hydrogen cyanide, ammonia, and methyl fluoride. For hydrogen cyanide, the dissertation presents predicted energies for all of the vibrationally excited states with up to four quanta of excitation distributed among the four modes. The root-mean-square error is 8.7 cm{sup {minus}1} for the states below 11,000 cm{sup {minus}1}. The force constants for ammonia are adjusted to reproduce the fundamental frequencies of ND{sub 3}. The force constants then predict the energies of states below 7000 cm{sup {minus}1} with an rms error of 5.8 cm{sup {minus}1} for ND{sub 3} and 16.7 cm{sup {minus}1} for NH{sub 3}. Finally, the adjusted force constants for methyl fluoride predict the energies of states below 4100 cm{sup {minus}1} with an rms error of 4.3 cm{sup {minus}1}. These force constants are also used to predict the CH stretching overtone region of CH{sub 3}F and the first, second and third overtone regions of CD{sub 2}FH for which experimental information is not available.

• Research Organization: Texas Univ., Austin, TX (USA)
• OSTI ID: 5002247
• Resource Relation: Other Information: Thesis (Ph. D.)
• Country of Publication: United States
• Language: English

Related Subjects

73 NUCLEAR PHYSICS AND RADIATION PHYSICS
99 GENERAL AND MISCELLANEOUS//MATHEMATICS, COMPUTING, AND INFORMATION SCIENCE
POLYATOMIC MOLECULES
VIBRATIONAL STATES
AMMONIA
CALCULATION METHODS
DATA ANALYSIS
MATHEMATICAL MODELS
METHYL FLUORIDE
THEORETICAL DATA
DATA
ENERGY LEVELS
EXCITED STATES
FLUORINATED ALIPHATIC HYDROCARBONS
HALOGENATED ALIPHATIC HYDROCARBONS
HYDRIDES
HYDROGEN COMPOUNDS
INFORMATION
MOLECULES
NITROGEN COMPOUNDS
NITROGEN HYDRIDES
NUMERICAL DATA
ORGANIC COMPOUNDS
ORGANIC FLUORINE COMPOUNDS
ORGANIC HALOGEN COMPOUNDS
653000* - Nuclear Theory
651000 - Nuclear Physics
990200 - Mathematics & Computers