Transcript n1BF3prsntnColumbusRK.ppt

Complementary Use of Modern
Spectroscopy and Theory in the Study of
Rovibrational Levels of
BF3
Robynne Kirkpatricka, Tony Masiellob, Alfons Weberc, and Joseph W.
Niblera
aDepartment
of Chemistry, Oregon State University
bPacific Northwest National Laboratory
cNational Institute of Standards and Technology, MD
Goals and Methods
• Push the limits of experiment to see how closely
ab initio methods model experiment
• Use isotopic substitution to gain additional
information about molecular potentials
How? Use modern, high resolution (0.0015 cm-1)
spectroscopy to study “simple” molecules of high
symmetry, such as BF3
Raman and IR active modes of group D3h
AX3 molecules
n1
A1 
n2
A2 (IR)
Exclusively Raman
Active
n3
E (R, IR)
n4
E (R, IR)
Consider SO3-- an intriguing
molecule!
n1 CARS
Q-Branch
34S16O
16
S3 O
32
3
S
16
O
3
32S16O
1065.5
1066.5
Raman Shift / cm-1
3
1067.5
n1 CARS Q-Branch
Q1 ≠ n1+DB1 J(J+1)+(DC1 - DB1)K 2 + higher terms
32 18
S O3
1067.5
32S18O
3
34 18
S O3
34S18O
1002.5
1003.5
1004.5
1005.5
Raman Shift / cm
1006.5
3
1007.5
-1
What causes this complex structure?
Perturbations to n1 (SO3) deduced using the
CARS Q-Branch
n1
A1'
2n4 (l=2) E '
2n4 (l=0) A1 '
n2 + n4 E '
2n2 A1 '
Fermi resonance
Coriolis
l-resonance
Let’s examine the CARS Q-Branches
of 10BF3 and 11BF3
CARS Experiment
n0
n0
Anti-Stokes
(AS) energy,
nS
nS
nA
nA
Vibrational
energy, ni
Induced dipole in sample ↔ Non-Linear
optical interaction
↔
a E + b E2 + c E(n0)E(n0)E(S)
CARS Intensity
n0
Sample
·Monitor CARS beam
·Scan Stokes beam
· Keep green beam at a constant
frequency
Experimental Setup
◦ Nd:YAG output locked to
single frequency
◦ Long pulse → Very high spectral
Computer
resolution (~0.001 cm-1)
Photodiode
Ar+ laser
I2
cell
Tunable Ring dye laser
Dye cell
Nd:
Dye cell
Dye cell
Amplification of Stokes beam
Sample
YAG
Filter
PMT
Integrator
Significant perturbations not evident for 10BF3 
CARS Q-Branch Spectra:
n1 mode of 10BF3
884.7
885.1
885.5
Raman Shift (cm )
-1
Predict structure according to:
Q1 = n1+DB1 J(J+1)+(DC1 - DB1)K 2 + higher terms
With intensities I ~ C g(J,K) (2J+1) exp[-hF0(J,K)/kT])
IR studies on BF3 (Masiello, Maki, Blake) give n1
parameters indirectly from various transitions:
n3
n1 + n
E'
A 2 ''
2
n1 +n
4
E'
n1
Energy
Ground
State
n1 Q-Branch of 10BF3
Calc.
Expt.
884.7
885.1
-1
885.5
Raman Shift (cm )
What do we predict for 11BF3?
Interesting Frequency Shift Observed with
Isotopic Substitution at the Center of Mass!
≈0.2 cm-1
10
BF3 Expt.
11
BF3 Expt.
884.5
884.9
885.3
Raman Shift (cm-1)
885.7
n1 Shift:
Due to an unrecognized Fermi resonance?
Due to changes in anharmonicity constants?
1
n = w + 2x + x + x + x
1
1
11 2 12 13 14
Answer these
questions by
making use of
►
IR data
►
ab initio calculations
Ask: How well do Measured xij’s and
isotopic shifts correspond to results of
ab initio (Gaussian 03) calculations?
► Instruct Gaussian 03 to compute anharmonicities
(and other ro-vibrational parameters) using the
anharm option and B3LYP/cc-pVTZ
Problem: anharm only works for asymmetric tops
Solution: Small distortion (0.0002 Å ) of one BF3 bond
Vibrational constants in cm
-1
10
11
for BF3 and BF3
10
constant
w1
x 11
x 12
x 13
x 14
n1
w1 - n1
(Hard to get)
(Easy to get)
11
BF3
exp.
theory
897.243 889.306
-1.158
-1.120
-3.374
-3.673
-4.479
-4.676
-3.115
-3.081
885.645 877.473
11.597
11.833
BF3
exp.
theory
897.327 889.306
-1.169
-1.120
-3.318
-3.621
-3.607
-3.765
-3.879
-3.818
885.843 877.673
11.483
11.633
n1(10BF3) - n1(11 BF3) -0.198
-0.200
n 1 = w1 + 2 x11 + 1 x12 + x13 + x14
2
exp.
theory
What about other anharmonic
shifts?
Anharmonic shifts (cm-1)
10BF
constant
Exp.
3
B3LYP/
Exp.-calc
% diff
cc-pVTZ.
w1-n1
11.6
11.8
-0.2
-2.0
w2-n2
4.1
4.1
0.0
-1.0
w3-n3
25.2
25.6
-0.4
-1.5
w4-n4
2.9
2.8
0.1
3.1
Conclusion: theory gives excellent
values for anharmonic shifts!
Vibration-rotation constants in cm-1 for 10BF3
Theory
%Diff
0.346
0.342
1.2
a1 ´ 103
0.685
0.676
1.2
a2 ´ 103
-0.119
-0.138
-16.4
a3 ´ 103
1.511
1.512
0.0
a4 ´ 103
-0.509
-0.513
-0.7
0.173
0.171
1.2
a1 ´ 103
0.343
0.338
1.4
a2 ´ 103
-0.281
-0.291
-3.7
a3 ´ 103
0.889
0.867
2.5
a4 ´ 103
0.108
0.089
18.0
z33z
0.777
0.812
-4.5
z44z
-0.806
-0.812
-0.7
Constant
Exp.
Be
Ce
Coriolis constants
Bv = Be – Si ai (vi+ di )+ higher terms
Fv = Bv J ( J + 1) + (Cv  Bv ) K 2  2C v z i K
Rotational distortion constants (cm-1) for ground
state of 10BF3
Exp.
Theory
DJ x 107
4.303
4.243
DJK x 107
-7.593
-7.471
% diff
1.4
1.6
DK x 107
3.570
3.482
2.5
HJ x 1012
1.332
1.335
HJK x 1012
-5.089
-5.154
HKJ x 1012
6.190
6.311
-0.2
-1.3
-1.9
HK x 1012
-2.432
-2.490
-2.4
Since parameters are well-determined by theory, can we ab
initio calcs. to accurately assess the potential surface?
kii ↔ ni
kiii , kiiii ↔ a, z, xii
2
3
4
Vi = kii Q i + kiii Q i + kiiii Q i + ...
We can be confident such higher order terms in the potential
are well-defined by ab initio calculations.
10BF
mode
11BF
3
3
kii
kiii
kiiii
Kii
kiii
kiiii
1
889.3
-23.7
0.8
889.3
-23.7
0.8
2
711.4
---
1.3
683.5
---
1.2
3
1511.5
52.0
4.3
1457.9
49.2
4.1
4
476.9
4.2
0.4
475.0
4.3
0.4
Out-of-plane bend
Symmetric BF stretch
V = 711.4 Q22 + 0 Q23 + 1.3 Q24
800
800
600
600
400
400
V/cm
-1
V/cm-1
V = 889.3 Q12 - 23.7 Q13 + 0.8 Q14
200
200
0
0
-1
-0.5
0
-200
-400
Q1
0.5
Cubic
(100x)
Quartic
(100x)
1
-1
-0.5
0
-200
-400
Q2
0.5
Quartic
(100x)
1
In-plane bend
Anti-symmetric BF stretch
V = 476.9 Q42 + 4.2 Q43 + 0.4 Q44
800
800
600
600
400
400
V/cm
-1
V/cm-1
V = 1511.5 Q32 + 52.0 Q33 + 4.3 Q34
200
200
0
-1
-0.5
Cubic
(100x)
0
0
-200
-400
Q3
0.5
Quartic 1
(100x)
-1
-0.5
Cubic
(100x)
0
-200
-400
Q4
0.5
Quartic
(100x)
1
Conclusions
● CARS spectra of BF3 confirm validity of n1
parameters deduced indirectly from IR studies
● 11n1 - 10n1 shift reproduced by ab initio
calculations
● BF3 parameters (D’s, H’s, a’s, x’s, z’s, …) in
excellent agreement with ab initio anharmonic
values
● Results indicate theory can give very useful
estimates of higher-order parameters needed
for the analysis of complex ro-vibrational
spectra.