Transcript 07Col2MMA.ppt

Remeasurement* of the Microwave Spectrum of
2-Methylmalonaldehyde and Analysis of the
Hydrogen Transfer and Internal Rotation Motions
Vadim V. Ilyushin,1 Eugene A. Alekseev,1
Yung-Ching Chou,2,3 Yen-Chu Hsu,2
Jon T. Hougen,4 Frank J. Lovas,4 and Laura B. Picraux5
1Institute
of Radio Astronomy of NASU, Chervonopraporna 4, 61002 Kharkov, Ukraine
2Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 107, Taiwan
3Department of Natural Science, Taipei Municipal Univ. of Education, Taipei, Taiwan
4Optical Technology Division, NIST, Gaithersburg, MD 20899-8441, USA
5Chemical Sciences and Technology Laboratory, NIST, Gaithersburg, MD 20899, USA
*Previously presented at Columbus 2005
Two large-amplitude motions :
Intramolecular hydrogen transfer
Internal rotation of a methyl rotor
O8
H10
C5
H2
H3
H11
C6
C12
O7
C4
H1
O8
(123)(45)(78)(9,10)
H9
H10
C5
H2
H11
C6
C12
O7
C4
H9
H1
H3
Intramolecular hydrogen transfer induces a
tautomerization in the ring, which then triggers a
60 degree internal rotation of the methyl rotor.
Part of Columbus 2005 Slide
Refit of Sanders’ data
(0  J 10, 0  K  6, 14    40 GHz)
N. G. Sanders, J. Mol. Spectrosc. 86, 27-42 (1981)
We used G12m group-theoretical formalism developed for
the wagging-torsional-rotational problem in methylamine
to refit Sanders’ spectra (2-MMA-d0 and -d1)
# of rovibrational levels in fit
# of parameters
r.m.s. deviation ()
# of lines with deviation > 3
# of Sanders' lines excluded
2-MMA-d0
87
14
0.12 MHz
2
7
2-MMA-d1
75
10
0.10 MHz
none
3
Today: 30  more data, 10-20  more precision
Overview of 2-MMA measurements used in the present work
Lab
Year Apparatus
Range Unc #lines d0 rms #lines d1 rms
[GHz] [kHz]
NIST
[kHz]
2005 FTMW 18-24 4,10 176
Harvard 1980 Stark
14-40
50
[kHz]
3.0
161 9.2
68 48.0
53 32.5
Kharkov 2006 sub-mm 49-149 10 1876 10.9 1976 11.7
Kharkov 2006 sub-mm 49-149 50
458 21.0
362 26.7
Overall rms of 2578 d0 lines to 37 parameters = 15.1 kHz
Overall rms of 2552 d1 lines to 32 parameters = 15.3 kHz
Overall weighted rms (dimensionless) = 0.98 and 1.08.
Spectral fit to tunneling Hamiltonian is excellent.
So what is the problem? = Today’s Talk
Interpretation: Fitting Parameters → Barrier Heights
This is a long-standing problem (last 20 years)
for the multi-dimensional tunneling formalism.
Assumption: To determine the barrier height from
tunneling splittings for a 1-D tunneling path we need:
1. tunneling splitting (from the fitting)
2. path length a between the two minima 0  x  a
3. potential function along this path
V(x)
4. effective mass moving along this path m(x)
For this molecule there are two tunneling frequencies
h2v = H-transfer + 60º int. rot. tunneling frequency
(→ NH2 inversion frequency for CH3NH2)
h3v = pure 120º internal rotation tunneling frequency
So we want two barriers.
The calculation of both barriers leads to troubles:
The internal rotation barrier has a small trouble.
The H-transfer barrier has a big trouble.
Consider first the pure internal rotation problem
Use formalism in the literature (Lin and Swalen)
H = F p2 + ½ V3(1-cos3)
Calculate F from structure and fix it
Determine V3 from A-E tunneling splitting
for both –OH and –OD isotopologs of 2-MMA
Check for consistency: V3(OH) = 399 cm-1
V3(OD) = 311 cm-1
d1 anomaly: 3-fold increase in torsional splitting (h3v)
MHz
h 3v
r
A-(B+C)/2
(B+C)/2
(B-C)/4
s1
2-MMA-d0
2-MMA-d1
-111.494(6)
0.031921915(2)
2236.8475(3)
2801.13540(8)
353.49188(4)
-348.213(7)
0.0318086(9)
2253.916(4)
2755.781(1)
344.5959(6)
29.389(4)
47.06(4)
1. Not an assignment or fitting problem (this work)
2. Path1-Path2 information leakage in model ?
3. O-H stretch zero-point effect on V3 ? (ab initio?)
Next look at barrier to H transfer motion
from a 1-Dimensional point of view:
Tunneling path is a 1-D line in (3N-6)-D space
Use a 1-D tunneling coordinate  with a
6-fold periodic potential and path dependent F
H = F() p2 + ½ V6(1-cos6)
Determine F() = (constant)/I() classically
T = ½ i mi vi2 = ½ i mi (dri/dt)2 =
= ½ i mi (dri/d)2 (d/dt)2
= ½ i I() (d/dt)2
Two large-amplitude motions :
Intramolecular hydrogen transfer
Internal rotation of a methyl rotor
O8
H10
C5
H2
H3
H11
C6
C12
O7
C4
H1
O8
(123)(45)(78)(9,10)
H9
H10
C5
H2
H11
C6
C12
O7
C4
H9
H1
H3
Intramolecular hydrogen transfer induces a
tautomerization in the ring, which then triggers a
60 degree internal rotation of the methyl rotor.
Barrier results
Move hydroxyl and methyl H’s = 4 H’s
V3(OH) = 413 cm-1
V3(OD) = 730 cm-1
Move only hydroxyl H = 1 H
V3(OH) = 4056 cm-1
V3(OD) = 4064 cm-1
Mass of the 3 methyl H’s is “hidden”
during the tunneling process.
What does this mean ???
Maybe the H-transfer dynamics are really
a “2-D problem”
H. Ushiyama & K. Takatsuka (ab initio),
Angew. Chem. Int. Ed. 44 (2005) 1237 say
First comes the H transfer
Then comes the electron rearrangement
= single double bond rearrangement
Then comes corrective internal rotation
of the CH3 group
Which implies no time reversal symmetry
hydrogen transfer
The six equivalent local minima in the “hydrogen
transfer-methyl torsion” potential surface
n=2
n=6
n=4
n=6

-120
-60
0
60
120
180
240
300
 (degree)
360

n=1
n=3
n=5
methyl torsion
Ground vibrational level  A1, B1, E1, E2 sublevels
2-dimensional particle-in-a-box tunneling problem to
imitate the 2-MMA H-transfer tunneling problem
Potential energy surface with
inversion symmetry V(x,y) = +V(-x,-y)
+
V= 
V=0
H-transfer
+a
V=W
a
V= 


torsion
V=0
0
+
2-MMA
CH3NH2
CH3NH2
Energy
vt=1
-OD -OH
vt=0
E2
h3v
B1
2h2v
E2
h2v
E1
0
0
B1
+2h3v
0
-1.0
A1
H-transfer
 h3v
E1
+h2v
A1
+2h2v
-0.5
h2v 
-1.0
-0.5 internal rotation 0
6. Related molecules:
Ref. 15
5-methyltropolone
Ref. 16
5-methyl-9-hydroxyphenalenone
acetic acid–benzoic acid mixed dimer
Ref. 17
15. Nishi, Sekiya, Kawakami, Mori, Nishimura, JCP 111, 3961 (1999)
16. Nishi, Sekiya, Mochida, Sugawara, Nishimura, JCP 112, 5002 (2000)
17. Nandi, Hazra, Chakraborty, JCP 121, 7562 (2004)