Transcript neop.pptx

67th OSU International Symposium on Molecular Spectroscopy
TH14
Rotational spectrum of neopentyl
alcohol, (CH3)3CCH2OH
Zbigniew Kisiel, Lech Pszczółkowski,
Institute of Physics, Polish Academy of Sciences,
Warszawa, Poland
Zhifeng Xue, Martin A. Suhm
Institut für Physikalische Chemie, Universität Göttingen
Göttingen, Germany
Neopentyl alcohol: expectations from ab initio
trans
a = 0.17 D
b = 1.54 D
c = 0
gauche
a = 1.28 D
b = 0.81 D
c = 0 ?
DE  +1 kJ/mol
Measurement strategy:
 The room temperature MMW spectrum was measured first in order to
reach an assignment
 MMW measurements were eventually extended to 90 – 278 GHz with
continuous coverage of :
1) 97.5 – 136 GHz
2) 182 – 243 GHz
 Assignment allowed commencing supersonic expansion FTMW
measurements for both conformers, which were carried out at 5 – 16 GHz
 Observed transition types:
trans:
bR, bQ,
gauche:
aR
also aR but only in FTMW
only, while bR predicted but not yet assigned
The a-type supersonic expansion spectrum of NPA:
Both conformers observed but the gauche spectrum only visible with He
carrier gas (not Ar), hence DE(gauche-trans) probably < 20 cm-1
J = 2  1:
Ka = 1
0
0
1
B-C  1 MHz
trans
a = 0.17 D
MHz
A selection of bR-type transition sequences for trans-NPA:
Ka = 10
Ka = 2120
Kc = 10
The J = 46  45 a-type band for gauche-NPA:
Since in the gauche B-C  1 MHz the asymmetry splitting is very small and
affects only a limited number of transitions
17
16
15
10
5 3
2
Ka = 1
0
2
1
Measurements and assignment of the rotational spectrum:
The ground state a-type band in the room-temperature gauche spectrum is
partially overlapped by similar bands for vibrationally excited states
Spectroscopic constants for NPA
+ octics
Assignment of two similar excited vibrational states
140 MHz
A
B
g.s., trans
Spectroscopic constants for the vibrational satellites:
Satellite A has some intriguing properties:
- quartics are similar to those of gauche (but selection rules of the trans)
- almost no need for sextics and well behaved Q-types
bQ-type
transitions near 102 GHz for Ka = 29←28:
The Q-type transitions for satellite A are well described by a single state fit, and
also require a lowest order Hamiltonian than for other states.
g.s., trans
B
A
Clues to DE: resonances in gauche-PMA at Ka= 16 and 17
Clues to DE: resonances in gauche-PMA at Ka= 20 and 21
Perturbation in a symmetric top K-progression:
 Similar perturbation was observed among excited states of CH3CCl3 and was eventually
fitted with a Coriolis coupling Hamiltonian yielding, among others, a precise value of DE
for the interacting states
 The global solution was critically dependent on the parameters and its unambiguous
location required extensive measurements
Kisiel, Pszczolkowski, Cazzoli, Dore, J.Mol.Spectrosc. 251, 235 (2008)
SUMMARY:
 The rotational spectrum of both the trans and the gauche species of neopentyl
alcohol has been assigned
 The rotational and quartic centrifugal distortion constants for both conformers are
consistent with ab initio predictions
 Supersonic expansion spectra indicate that gauche is the less stable species but
probably by not more than 20 cm-1
 No torsional splitting in either trans or the gauche ground states
 Several perturbations observed in the compact, a-type bands for the gauche
species may allow determination of DE0(gauche-trans), but it is still necessary to
identify perturbed transitions in the trans conformer and to reach a satisfactory
coupled fit
 For the trans conformer two excited vibrational satellites at 100 cm-1 assigned:
somewhat puzzling single state constants, but these are most likely v = 1
substates of the HOCH2C(CH3)3 torsion