View
215
Download
0
Category
Preview:
Citation preview
Analysis of the microwave spectrum of the three-top molecule trimethoxylmethane
L. Coudert,a G. Feng,b and W. Caminatib
aLaboratoire Interuniversitaire des Systèmes Atmosphériques, Créteil, FrancebDipartimento de Chimica “G. Ciamician,” Universita de Bologna, Bologna, Italy
The three conformers
TMM1
C1 symmetry
TMM2
C3 symmetry
375 cm
TMM3
Cs symmetry
316 cm
14 transitions
Overview
• The internal rotation problem in trimethoxylmethane
• The Schrodinger equation
• Torsion-rotation energy levels
• Calculated tunneling patterns
• Observed tunneling patterns
• Analysis
Internal rotation in trimethoxylmethane
Trimethoxylmethane displays internal rotation of its
three inequivalent methyl groups.
Torsion-rotation energy levels should be obtained
solving a 3-D Schrödinger equation.
The model
Assumptions:
• 4 rigid parts: frame HCO3 + the 3 methyl groups.
• The 3 axes of internal rotation are fixed.
• Each methyl group has C3v symmetry.
• Each axis of internal rotation is parallel to the C3 axis of symmetry.
The exact Hamiltonian
Describing the internal rotation of each methyl group
with the angles α1, α2, and α3, the Hamiltonian is:1
1. Ohashi, Hougen, Suenram, Lovas, Kawashima, Fujikate, and Pyka, JMS 227 (2003) 28
Energy level calculation
Calculation is carried with a DVR approach.1,2
With a usual FIR basis set: [21 x 21 x 21]2 = 9261 x 9261
With a DVR basis set: [21 x (2J+1) x 3]2
1. Ligth and Carrington, Adv. Chem. Phys. (2003)2. Lee and Tuckerman, J. Phys. Chem. A 110 (2006) 5549
FIR and DVR functions for C3 symmetry
Potential energy function
1
2
3
1. Ab initio calculation at the MP2/6-311++G** level
Solving the 3-D Schrödinger equation
H(α1,α2,α3)
H(α1=αp,α2,α3)
H(α1=αp,α2=αq,α3)
HR(α1=αp,α2=αq,α3=αn)
α1 active coordinate
α2 active coordinate
α3 active coordinate
1. Lauvergnat, Nauts, Justum, and Chapuisat, J. Chem. Phsy. 114 (2001) 65922. Light and Bacic, J. Chem. Phys. 87 (1987) 4008
Tunneling energy level diagram
Ohashi, Hougen, Suenram, Lovas, Kawashima, Fujikate, and Pyka, JMS 227 (2003) 28
Total number of level is 27
Statistical weights
Total statistical weight is 29
Calculated tunneling pattern
Observed tunneling patterns
Analysis
The analysis should yield:
•height or the three barriers hindering the internal
rotation
•direction cosine of the axes of internal rotation in the
molecule-fixed-axis system
•assignment of the tunneling component is an issue
FBR and DVR functions
The FBR and DVR functions depend on the problem you want to
solve and on the associated Gaussian quadrature.1,2
Non-rigid molecule displaying internal rotation1,2 parameterized by
the angle α
1. Ligth and Carringyon, Adv. Chem. Phys. (2003)2. Lee and Tuckerman, J. Phys. Chem. A 110 (2006) 5549
No
symmetry
N
DVR functions189 246 34195
Cs symmetry
17111727 16210854
N
C3 symmetry
These DVR functions will be used
in the present investigation.
Recommended