Barrier to asymmetric internal rotation, conformational stability, vibrational spectra and assignments, and Ab Initio calculations of n ‐butane‐ d 0 , d 5 and d 10
The asymmetric torsional potential function of n ‐butane has been redefined based on a combination of results obtained from experimental and theoretical calculations. From the vibrational spectra of gaseous n ‐butane below 500 cm −1 the asymmetric torsional fundamentals of the more stable s‐trans an...
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Veröffentlicht in: | Journal of Raman spectroscopy 1991-11, Vol.22 (11), p.683-704 |
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Sprache: | eng |
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Zusammenfassung: | The asymmetric torsional potential function of
n
‐butane has been redefined based on a combination of results obtained from experimental and theoretical calculations. From the vibrational spectra of gaseous
n
‐butane below 500 cm
−1
the asymmetric torsional fundamentals of the more stable
s‐trans
and high‐energy
gauche
conformations of
n
‐butane have been assigned at 121.28 and 116.60 cm
−1
, respectively, each with excited states falling to lower frequencies. Additionally, from variable‐temperature studies of the Raman spectra for
n
‐butane‐
d
0
and
d
10
, the conformational enthalpy difference has been determined to be 381 ± 41 cm
−1
(1089 ± 117 cal mol
−1
) and 369 ± 72 cm
−1
(1055 ± 206 cal mol
−1
), respectively. Optimized structural parameters, obtained from
ab initio
methods, for the
s‐trans
and
gauche
conformers have been used to allow for structural relaxation during the asymmetric internal rotation and to place constraints on the determined potential function. The resulting potential coefficients are
V
1
= 584 ± 7,
V
2
= ‐96 ± 7,
V
3
= 1165 ± 3,
V
4
= 45 ± 3,
V
5
= ‐3 ± 2 and
V
6
= ‐40 + 1 and the
s‐trans
to
gauche, gauche
to
gauche
and
gauche
to
s‐trans
barriers are determined to be 1274, 1370 and 891 cm
−1
, respectively, with an enthalpy difference between the conformers of 383 ± 17 cm
−1
(1095 ± 49 cal mol
−1
). This potential function is consistent with a dihedral angle for the
gauche
conformer of 65.4°. Additionally, this potential has a
‘syn’
barrier, which is the energy difference between the
s‐trans
minimum and the
gauche
to
gauche
transition state, of 1753 cm
−1
(5.01 kcal mol
−1
), which is in excellent agreement with the most recent value of 1836 cm
−1
(5.25 kcal mol
−1
) obtained from a very high‐level
ab initio
calculation. In order to provide a more complete description of the normal vibrational modes of
n
‐butane, normal coordinate analyses have been completed for
n
‐butane‐
d
0
, ‐
d
5
and ‐
d
10
based on force constants from
ab initio
calculations. The infrared (3500‐40 cm
−1
) and Raman (3500‐10 cm
−1
) spectra of
n
‐butane‐
d
5
and ‐
d
10
have been obtained and complete vibrational assignments are provided. The structural parameters, conformational stabilities, barriers to internal rotation and vibrational frequencies which have been determined experientally are compared with those obtained by
ab initio
methods. |
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ISSN: | 0377-0486 1097-4555 |
DOI: | 10.1002/jrs.1250221115 |