Transcript mss_sibert_2013.pptx

Alkyl CH Stretch Vibrations as a Probe
of Conformational Preferences
Evan Buchnanan and Timothy Zwier and Ned Sibert
Late in the Day on June 19th at the OSU MSS
Funded by NSF
[2,2,2]-Para-Tricyclophane CH Stretch Region
David Perry
Let’s start with smaller molecules.
Fitting Spectra
Start with a zero order Hamiltonian and dipole
N
H (0)
1
  i (ni  )
2
i 1
N
( 0)   i qi
i 1
Add some Fermi coupling terms
H (1)   f ikl (ai a j ak  h.c.)
i , j ,k
Pick a basis, diagonalize the H matrix, and calculate the spectrum.
Normal Modes vs Local Modes
for H(0)
Normal modes are sensitive to small potential changes.
Local modes are insensitive to small potential changes.
Local modes potentials are transferrable.
The local mode picture is approximate.
Van Vleck Perturbation Theory
H = HS-S + HO-O + HS-O
VVPT
H
dr
H
dr
S-S
H
dr
O -O
Van Vleck Perturbation Theory
VVPT
All calculations follow from a
Gaussian Z-matrix input file.
E. L. Sibert, Molecular Physics, 2013.
VV Results for CH2X-CH2X molecules
au
bg
(a)
bu
ag
Construct Hamiltonians that
describe the stretch coupling
b
a
a
b
(b)
au
bg
(a)
bu
ag
Construct Hamiltonians that describe
the stretch bend couplings
b
a
a
b
(b)
au
bg
bu
ag
Fit H parameters to a single conformer
(a)
au
bg
(a)
bu
ag
b
a
a
b
(b)
Probing molecular structure with CH chromophores
with the Zwier Group at Purdue
Our goal is to develop theoretical models that will enable us to use the CH stretch as a
probe of environment. Initial work focuses on developing Hamiltonians that allow us
to predicting spectra of the molecules based on input from electronic structure theory
and couplings that are scaled to the DPE C2h spectrum.
DPE C2h
DPE C2
DPOE C2h
DPOE C2
so
so/ss
ss/so
so/ss
ss
as
as
ss/so
as
as
ss/so
as so/ss
au
bg
(a)
bg
bu
ag
a
b
(c)
bu
ag
Construct Hamiltonians that
describe the stretch bend
couplings
b
a
au
(b)
(d)
b
a
a
b
so
so/ss
ss/so
so/ss
ss
as
as
ss/so
as
as
ss/so
as so/ss
Concluding Remarks
• Developed a model for CH2 stretches.
• Hamiltonian has parameters that are fit to one
member of a group of similar species.
• Working to extend to CH3 groups.
• Working to better parameterize the
Hamiltonian.
• Would like to add waters.