Vibrational Analysis of Small Molecules
Methodology
MNDO/AM1/PM3 Semiempirical MO Theory
Ab Initio MO Theory
Techniques Used
Building, Optimization, Analysis of
Bonding Parameters, Output Analysis
Abstract. You will build, optimize, and carry out a force constant
vibrational calculation on formaldehyde (H2C=O). You will be
able to compare the computed results to the experimental numbers for this
molecule.
Procedure. Build and optimize formaldehyde, using a semiempirical
method, an STO-3G computation, and a 6-31G* computation. Visualize and identify
the various vibrational modes. In your Results table, record the symmetries,
frequencies and atomic motions of the vibrational modes at each level of
theory, and compare to the experimental numbers listed below.
Benchmarks. The experimental IR spectrum of formaldehyde is shown
below, along with the frequencies of major IR active modes.
Copyright by Paul M. Lahti 1995.