Absolute configurations, predominant conformations, and tautomeric structures of enantiomeric tert-butylphenylphosphinothioic acid

Article abstract of DOI:10.1021/jo0107406

Vibrational absorption and circular dichroism spectra of dextrorotatory, levorotatory, and racemic mixture of tert-butylphenylphosphinothioic acid have been measured in CCl4 solutions in the 2000-900 cm^-1 region. The conformations for both tautomeric structures of (S)-tert-butylphenylphosphinothioic acid are investigated using the B3LYP functional with the 6-31G* basis set. For the most stable conformation, the absorption and VCD spectra are predicted ab initio using the B3LYP functional with 6-31G*, 6-311G(2d, 2p), 6-31+G*, and 6-311G(3df, 3pd) basis sets. A different functional, B3PW91, was also used with the 6-31G* basis set. The predicted spectra are compared to the experimental spectra. The comparison indicates that (-)-tert-butylphenylphosphinothioic acid is of the (S)-configuration and exists in only one tautomeric structure and one conformation in CCl₄ solution.

Full text of DOI:10.1021/jo0107406

J . Org. Chem. 2001, 66, 9015-9019
9015
Absolu te Con figu r a tion s, P r ed om in a n t Con for m a tion s,
a n d Ta u tom er ic Str u ctu r es of En a n tiom er ic
ter t-Bu tylp h en ylp h osp h in oth ioic Acid
Feng Wang and Prasad L. Polavarapu*
Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235,and
J o´ zef Drabowicz, Marian Mikołajczyk, and Piotr Ły z˘ wa
Center of Molecular and Macromolecular Studies, Polish Academy of Sciences, 93-236 Ł o´ d z´ ,
Sienkiewicza 112, Poland
Prasad.L.Polavarapu@vanderbilt.edu.
Received J uly 23, 2001
Vibrational absorption and circular dichroism spectra of dextrorotatory, levorotatory, and racemic
mixture of tert-butylphenylphosphinothioic acid have been measured in CCl
4
solutions in the 2000-
-1
9
00 cm region. The conformations for both tautomeric structures of (S)-tert-butylphenylphosphi-
nothioic acid are investigated using the B3LYP functional with the 6-31G* basis set. For the most
stable conformation, the absorption and VCD spectra are predicted ab initio using the B3LYP
functional with 6-31G*, 6-311G(2d, 2p), 6-31+G, and 6-311G(3df, 3pd) basis sets. A different
functional, B3PW91, was also used with the 6-31G* basis set. The predicted spectra are compared
to the experimental spectra. The comparison indicates that (-)-tert-butylphenylphosphinothioic
acid is of the (S)-configuration and exists in only one tautomeric structure and one conformation in
CCl
4
solution.
In tr od u ction
confirmed using vibrational circular dichroism (VCD).11
(
+)-tert-Butylphenylphosphinothioic acid was assigned
Chiral organophosphorus compounds, especially phos-
phines, are extensively used in asymmetric and stereo-
selective synthesis,1 and synthesis and characterization
of phosphines have attracted much attention in recent
years. In a rich family of enantiomerically pure organo-
phosphorus derivatives, the enantiomeric tert-butylphe-
nylphosphinothioic acids have recently been used as
versatile chiral solvating agents for the NMR determi-
8
the (R)-configuration by chemical correlation, and this
assignment has not yet been confirmed by spectroscopic
methods. (R)-(+)- and (S)-(-)-enantiomers of tert-bu-
tylphenylphosphinothioic acid have been studied using
electronic CD spectra of their complexes with tetraac-
-3
12
etatomolybdenum. The possible tautomeric structures
or conformations in each of the tautomeric structures of
tert-butylphenylphosphinothioic acid, however, were not
analyzed or determined in the literature.
nation of the enantiomeric excesses of a large number of
chiral organic and heteroorganic compounds.4
-6
tert-
Some theoretical studies on simple phosphine oxides,
Butylphenylphosphinoselenoic acid has also been noted
3 2
P(OH) , OPH(OH) , and phosphorothioate anions have
been reported. tert-Butylphenylphosphinothioic acid 1,
like secondary phosphine oxides, phosphites, and phos-
phorothioate anions, can exist in an equilibrium be-
tween different tautomeric structures 1A and 1B. How-
ever, the stability of the different tautomeric forms of 1
has not been studied before, and the equilibrium constant
is unknown.
to behave similarly.7
13
tert-Butylphenylphosphinothioic acid can be prepared
8
from tert-butylphenylphosphine oxide. (-)-tert-Butylphe-
14
nylphosphine oxide was assigned the (S)-configuration
by chemical correlation,8
-10
and this assignment was
*
Corresponding author. Fax: (615) 322-4936.
(1) Pietrusiewicz, K. M.; Zabłocka, M. Chem. Rev. 1994, 9, 1375.
2) (a) Noyori, R. Asymmetric Catalysis in Organic Synthesis; J ohn
(
Wiley & Sons: New York, 1994. (b) Quin, L. D. A Guide to Organo-
phosphorus Chemistry; Wiley-Interscience: New York, 2000. (c) Cata-
lytic Asymmetric Synthesis; Ojima, I., Ed.; Wiley-VCH: New York,
2
000.
3) Drabowicz, J .; Ły z˘ wa, P.; Omela n´ czuk, J .; Pietrusiewicz, K. M.;
Mikołajczyk, M. Tetrahedron: Asymmetry 1999, 10, 2757.
4) Drabowicz, J .; Dudzi n´ ski, B.; Mikołajczyk, M. Tetrahedron:
Asymmetry 1992, 3, 1231.
5) Omela n´ czuk, J .; Mikołajczyk, M. Tetrahedron: Asymmetry 1996,
, 2687.
6) Drabowicz, J .; Dudzi n´ ski, B.; Mikołajczyk, M.; Colonna, S.;
Gaggero, N. Tetrahedron: Asymmetry 1997, 8, 2267.
7) Drabowicz, J .; Omela n´ czuk, J . Mikołajczyk, M. Unpublished
results.
(
(
(
The determination of absolute configuration by chemi-
cal methods is based on the assumption of the steric
course of a given reaction. VCD is an independent
7
(
(
(
8) Michalski, J .; Skrzypczy n´ ski, Z. J . Organomet. Chem. 1975, 97,
(11) Wang, F.; Polavarapu, P. L.; Drabowicz, J .; Mikołajczyk, M. J .
Org. Chem. 2000, 65, 7561-7565.
(12) Omela n´ czuk, J .; Snatzke, G. Angew. Chem., Int. Ed. Engl. 1981,
20, 786.
C31-C32.
(
9) Skrzypczy n´ ski, Z.; Michalski, J . J . Org. Chem. 1988, 53, 4549.
(10) (a) Harger, M. J . P. J . Chem. Soc., Perkin Trans. 2 1978, 326.
(
b) Haynes, R. K.; Freeman, R. N.; Mitchell, C. R.; Vonwiller, S. C. J .
(13) (a) Chesnut, D. B. Heteroat. Chem. 2000, 11, 73. (b) Liang, C.;
Allen, L. C. J . Am. Chem. Soc. 1987, 109, 6449.
Org. Chem. 1994, 59, 2919.
1
0.1021/jo0107406 CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/20/2001

9
016 J . Org. Chem., Vol. 66, No. 26, 2001
Wang et al.
spectroscopic method without dependency on such an
assumption to determine the absolute configuration and
predominant conformations in the solution phase. Recent
developments in the application of density functional
theory1
5-17
and improvements in VCD instrumentation
make it possible to use VCD for a confident determination
of the absolute configurations and/or conformations in
1
8-19
solution phases.
Successful determinations of both
the absolute configurations and the predominant con-
1
8a
formations of 1,2,2,2-tetrafluoroethylmethyl ether,
1
8b
18c
18d
desflurane, epichlorohydrin, 3-butyn-2-ol, Troger’s
base,1 phenyloxirane, tert-butyl n-butyl sulfoxide,
9a
19b
19c
1
1
and tert-butylphenylphosphine oxide in the solution
phase are some recent examples that reflect the reli-
ability of VCD method. In addition, the combination of
ab initio and experimental methods can be used to study
chemical reactions and elucidate their equilibria.20
The equilibrium between tautomeric structures of tert-
butylphenylphosphine oxide and its dominant conforma-
tion has been determined earlier using theoretical and
experimental VCD spectra. Such information is not
available for tert-butylphenylphosphinothioic acid. There-
fore, we have measured the VCD of (-)-, (+)-, and (()-
tert-butylphenylphosphinothioic acid and undertaken the
state-of-the-art ab initio theoretical VCD investigations
using density functional method and different basis sets.
These results are used to elucidate the absolute config-
uration, predominant conformation, and tautomeric struc-
tures of chiral tert-butylphenylphosphinothioic acid.
F igu r e 1. Tautomeric structures and different conformations
of (S)-tert-butylphenylphosphinothioic acid. Compounds 1A
and 1B are different tautomeric structures; a and a ′ are
conformations around the C -C -P-Ct dihedral segment for
2
1
1A; b and b′ are conformations around the C
2
-C -P-Ct
1
dihedral segment for 1B; a 1-a 3 are conformations around the
Ct-P-O-H dihedral segment for 1A; and b1-b3 are confor-
mations around the Ct-P-S-H dihedral segment for 1B.
Resu lts a n d Discu ssion
butylphenylphosphinothioic acid. These starting geom-
etries converged to two conformations for each of the two
tautomeric forms, differing in the dihedral angles for the
The geometries were optimized at the B3LYP/6-31G*
level using the standard dihedral angles of 0, 60, 120,
Ct-P-C
1
2
-C segment (a and a ′ for tautomeric structure
1
80, 240, or 300° for the Ct-P-C
1
2
-C segment (where
1
A; b and b′ for tautomeric structure 1B), as summarized
Ct is the central carbon atom of tert-butyl group, see
Figure 1) for the two tautomeric structures of (S)-tert-
in Table 1 and shown in Figure 1. Because of the
symmetry of the benzene ring, the two different confor-
mations based on the dihedral angle C
each tautomeric structure have the same energies and
they cannot be distinguished. Keeping the Ct-P-C
segment in the gauche plus (labeled as a and b)
2
-C
1
-P-Ct for
(
14) (a) Stawi n´ ski, J . Handbook of Organophosphorus Chemistry;
Engel, R., Ed.; Marcel Dekker: New York, 1992; p 377. (b) Baraniak,
J .; Frey, P. A. J . Am. Chem. Soc. 1988, 110, 4059. (c) Frey, P. A.;
Reimsch u¨ ssel, W.; Paneth, P. J . Am. Chem. Soc. 1986, 108, 1720. (d)
Kabachnik, M. I.; Mastryukova, T. A.; Matrosov, E. I.; Fisher, B. Zh.
Strukt. Khim. 1965, 6, 691.
1
-
C
2
conformation, we studied the stability of the resulting
three conformations based on the difference in the
dihedral angles H-O-P-Ph and H-S-P-Ph of forms
(
15) (a) Becke, A. D. J . Chem. Phys. 1993, 98, 1372. (b) Becke, A. D.
J . Chem. Phys. 1993, 98, 5648.
16) Cheeseman, J . R.; Frisch, M. J .; Devlin, F. J .; Stephens, P. J .
Chem. Phys. Lett. 1996, 252, 211.
17) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
(
1
A and 1B (a 1-3 and b1-3 in Figure 1). Only one stable
(
conformation a 2 is obtained for tautomeric structure a ;
a 1 and a 3 conformations converged to a 2 (possibly due
to the interaction between the hydroxyl hydrogen atom
and the benzene ring and sulfur). For tautomeric struc-
ture 1B, three stable conformations are obtained. The
Robb, M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J . A.,
J r.; Stratmann, R. E.; Burant, J . C.; Dapprich, S.; Millam, J . M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J .;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J . B.; Cioslowski, J .; Ortiz, J . V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J .; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; J ohnson, B. G.; Chen,
W.; Wong, M. W.; Andres, J . L.; Head-Gordon, M.; Replogle, E. S.;
Pople, J . A. Gaussian 98, revision A.3; Gaussian, Inc.: Pittsburgh, PA,
converged C
2
-C
1
-P-Ct, C
2
-C
1
-P-O, C
2
-C -P-S, Ph-
1
POH, and PhPSH dihedral angles, optimized electronic
energies, Gibbs energies, and relative populations are
listed in Table 1. Tautomeric form 1A has a much lower
energy than tautomeric form 1B. On the basis of the
relative populations, we predicted the equilibrium con-
1
998.
18) (a) Polavarapu, P. L.; Zhao, C.; Ramig, K. Tetrahedron: Asym-
(
-
4
metry 1999, 10, 1099. (b) Polavarapu, P. L.; Zhao, C.; Cholli, A. L.;
Vernice, G. J . Phys. Chem. B 1999, 103, 6127. (c) Wang, F.; Polavarapu,
P. L. J . Phys. Chem. A 2000, 104, 6189. (d) Wang, F.; Polavarapu, P.
L. J . Phys. Chem. A 2000, 104, 1822.
stant to be 1.9 × 10 for the equilibrium between two
tautomeric structures of (S)-tert-butylphenylphosphi-
nothioic acid. Thus, for isolated (S)-tert-butylphenylphos-
phinothioic acid, the predominant tautomeric structure
is 1A and the conformation is a 2.
(19) (a) Aamouche, A.; Devlin, F. J .; Stephens, P. J . J . Am. Chem.
Soc. 2000, 122, 2346. (b) Ashvar, C. S.; Devlin, F. J .; Stephens, P. J .
J . Am. Chem. Soc. 1999, 121, 2836. (c) Drabowicz, J .; Dudzi n´ ski, B.;
Mikołajczyk M.; Wang, F.; Dehlavi, A.; Goring, J .; Park, M.; Rizzo, C.
J .; Polavarapu, P. L.; Biscarini, P.; Wieczorek, M. W.; Majzner, W. R.
J . Org. Chem. 2001, 66, 1122.
The converged tautomeric forms were found to have
potential energy minima (i.e., all vibrational frequencies
are real) at the B3LYP/6-31G* level. The absorption and
VCD intensities were calculated for conformations a 2 and
(20) Hehre, W. J .; Radom, L.; Schleyer, P. V. R.; Pople, J . A. Ab initio
Molecular Orbital Theory; J ohn Wiley & Sons: 1986.

Enantiomeric tert-Butylphenylphosphinothioic Acid
J . Org. Chem., Vol. 66, No. 26, 2001 9017
Ta ble 1. Con for m a tion s a n d En er gies of (S)-ter t-Bu tylp h en ylp h osp h in oth ioic Acid
starting geometrya
struct. C2C1PCt C2C1PO C2C1P S
converged geometrya
energyc
taut.
labelb
electronic
Gibbs
∆Ed pop. (%)
e
D
C2C1PCt C2C1PO C2C1PS
D
A
0
0
20
80
120
60
5
5
5
-120
-60
0
60
120
120
180
-120
-60
0
180
180
180
180
180
180
180
-60
60
94.7 -159.5 -35.8
94.7 -159.5 -35.8
94.7 -159.5 -35.8
102.2 a 2
102.2 a 2
102.2 a 2
102.2 a 2′
102.2 a 2′
102.2 a 2′
102.2 a 2
102.2 a 2
102.2 a 2
168.4 b1
51.1 b2
-1204.917683 -1204.726322
-1204.917683 -1204.726322
-1204.917683 -1204.726322
-1204.917683 -1204.726322
-1204.917683 -1204.726322
-1204.917683 -1204.726322
-1204.917683 -1204.726322
-1204.917683 -1204.726322
-1204.917683 -1204.726322
-1204.905865
0
0
0
0
0
0
0
0
0
100
6
1
1
-
-85.6
-85.6
-85.6
20.3
20.3
20.3
144.1
144.1
144.1
-
180
60
9
9
9
-160
-160
-160
-160
-160
-160
20
-36
-36
-36
-20
-20
-20
160
94.7 -159.5 -35.8
94.7 -159.5 -35.8
94.7 -159.5 -35.8
74.1 -160.2 -36.0
74.1 -160.2 -36.0
B
90
0
0
180
60
-60
0
9
9
-1204.902132
74.1 -160.2 -36.0 -84.3 b3
-1204.906337 -1204.718249 5.07
-
90
-60 -105.8
19.9
144.1 -84.3 b3′ -1204.906337 -1204.718249 5.07
a
b
Dihedral angles are given in degrees; D stands for PhPOH for the tautomeric form 1A and PhPSH for the tautomeric form 1B. See
Figure 1 for the labels. In Hartrees. Relative energy difference given in kcal/mol. e Percent population based on Gibbs energies.
c
d
F igu r e 2. Comparison of the experimental absorption spectra
of enantiomeric tert-butylphenylphosphinothioic acid at ∼0.34
F igu r e 3. Comparison of the experimental VCD spectra of
enantiomeric tert-butylphenylphosphinothioic acid at ∼0.34 M
in CCl4 solution (top three traces) with the predicted VCD
spectra (bottom six traces) of different stable conformations
of the (S)-configuration (structure and conformation labels in
Figure 1) obtained with different basis sets. The predicted
4
M in CCl solution (top three traces) with the predicted
absorption spectra (bottom six traces) of different stable
conformations of the (S)-configuration (structure and confor-
mation labels in Figure 1) obtained with different basis sets.
The predicted bands in the 1500-900 cm-1 region are num-
bered in the theoretical spectra as 1-36 in the decreasing
order of frequency. The corresponding experimental bands are
identified in the experimental spectra by these numbers. The
bands in the experimental spectra marked by asterisks are
considered to be due to impurities.
-1
bands in the 1500-900 cm region are numbered in the
predicted spectra as 1-36 in the decreasing order of frequency.
The corresponding experimental bands are identified in the
experimental spectra by these numbers.
spectrum. The notable differences are the following. (a)
The predicted frequencies of the bands for conformation
-
1
b3 at the B3LYP/6-31G* level. The predicted absorption
and VCD spectra were simulated with 5 cm-1 half widths
and Lorentzian band shapes. The theoretical spectra for
the predominant conformation a 2 can be compared to the
experimental spectra in Figures 2 and 3.
The experimental absorption spectra obtained for 0.34
4
M CCl solution are shown in Figure 2, where the
absorption spectrum of the solvent has been subtracted.
The absorption bands in the predicted spectrum (B3LYP/
a 2 in the range from 1220 to 800 cm are much larger
than the frequencies of the corresponding bands in the
experimental spectra. (b) The bands at 1083 (peak 21)
-
1
and 1045 cm (peak 23) in the predicted spectrum for
conformation a 2 are separated by a much larger gap than
-1
the corresponding bands around 920 cm in the experi-
mental spectra. (c) There are no bands corresponding to
-
1
the weak bands observed around 1020 cm
experimental spectra. Except for these differences, the
experimental spectra in CCl and solution are considered
to be in agreement with the predicted absorption spec-
in the
6
-31G*) for conformation a 2 show a one-to-one cor-
4
respondence with the absorption bands in experimental

9
018 J . Org. Chem., Vol. 66, No. 26, 2001
Wang et al.
trum for conformation a 2. In addition, since the predicted
absorption spectrum for conformer b3 does not cor-
respond well to the experimental absorption spectrum,
it is concluded that the tautomeric structure 1B does not
Con clu sion
The comparison of experimental and ab initio predicted
absorptions and VCD spectra indicates the following. (a)
-)-tert-Butylphenylphosphinothioic acid is of the (S)-
(
have a dominant population in CCl
The experimental VCD spectra obtained for 0.34 M
CCl solution are shown in Figure 3, where the VCD
4
solution.
configuration; the (-)-(S)-configuration is in agreement
with the chemical correlation reported in ref 8. (b) Only
one tautomeric structure and one conformation are
predominant for (-)-tert-butylphenylphosphinothioic acid
4
spectrum of the solvent has been subtracted. The sig-
nificant VCD bands in the predicted spectrum (B3LYP/
4
in CCl solution. (c) P-Chiral phosphines can be studied
6
-31G*) for the conformation a 2 of (S)-tert-butylphe-
using a combination of experimental and ab initio VCD
investigations because such molecules do show significant
VCD signals, but the accuracy of the calculated frequen-
cies needs to be improved when there are heavy atoms
in the molecule.
nylphosphinothioic acid are a negative-positive couplet
with a positive maximum at 1428 cm (peak 10) and a
negative maximum at 1474 cm
positive band at 1189 cm (peak 18), and a positive-
negative-positive triplet at 1083 (peak 21), 1045 (peak
-
1
-
1
(peaks 5 and 6), a
-
1
Exp er im en ta l Section
-1
2
3), and 1006 cm (peak 26). The same features are seen
Gen er a l. NMR spectra were recorded at 200 MHz.
in the experimental VCD spectrum for the (-)-tert-
butylphenylphosphinothioic acid except for the follow-
ing: (a) the relative intensities for the bands of the triplet
are different in the experimental VCD spectrum and (b)
there is no negative band in the experimental VCD
spectrum corresponding to the predicted negative band
at 1474 cm . These differences may come from the
remaining inaccuracy of the predicted relative intensities
and frequencies in calculation. The intensities of the
bands in the B3LYP/6-31G*-predicted VCD spectrum of
conformation b3 are very small compared to those
predicted for conformation a 2; also, the VCD sign pattern
predicted for b3 does not match the experimentally
observed sign pattern. Thus, as concluded earlier, the
population of tautomeric structure 1B is not expected to
Ra cem ic a n d En en tiom er ic ter t-Bu tylp h en ylp h osp h i-
2
1
n oth ioic Acid 1. Racemic thio acid 1 was prepared by the
addition of elemental sulfur to the sodium salt of racemic tert-
butylphenylphosphine oxide in methanol. The formed crude
sodium salts of the thio acid 1, dissolved in a water-methanol
mixture, were converted into the free acid 1 by acidification
of a water-methanol solution with diluted hydrochloric acid.
The formed thio acid 1 was extracted with chloroform. Evapo-
-
1
ration of the chloroform gave the chemically pure thio acid 1:
1
H NMR (CDCl
3
) δ 1.163 (d, J ) 10.4 Hz, 9H), 7.250-7.500
31
(
m, 3H), 7.720-7.830 (m, 2H); P NMR (CDCl ) δ 98.42 (s).
3
Optically active enantiomers of the thio acid 1 were prepared
via diastereomeric salts of the thio acid 1 with enantiomers of
optically active R-methylbenzylamine. The particular diaster-
eomers were formed in situ by the addition of elemental sulfur
to racemic tert-butylphenylphosphine oxide in the presence of
21
the optically active amine. The particular enantiomer of the
4
be significant in CCl solution.
thio acid 1 was liberated from the given diastereomeric salt
with optically active amine by acidification with hydrochloric
acid and had the following optical rotations: dextrorotatory
enantiomer [R]589 ) +27.3 (c 1.75, MeOH), ee ) 91.6%;
levorotatory enantiomer [R]589 ) -28.3 (c 1.20, MeOH), ee )
The discussion presented above for predicted spectra
pertains to those obtained at the B3LYP/6-31G* level.
To investigate the influence of basis set on the observed
differences between experimental and predicted spectra,
calculations were also undertaken with larger basis sets,
namely, 6-311G(2d, 2p), 6-31+G, and 6-311G(3df, 3pd),
using the same functional. The geometry of conformation
a 2 was optimized with each of these basis sets, and
vibrational absorption and VCD spectra were obtained
at the optimized geometries. These calculations were also
repeated with a different functional, namely, B3PW91,
using the 6-31G* basis set. All of these results are
presented, along with B3LYP/6-31G* results, in Figures
1
31
9
5%. The spectral data ( H and P NMR) were in full
agreement with those reported above for the racemic thio acid
1. Earlier, the racemic thio acid 1 was resolved into enanti-
omers via diastereomeric salts with enantiomers of R-methyl-
benzylamine.8
-10
1
8
Mea su r em en ts. The VCD spectra were recorded with 3
-
1
h of data collection time at a 4 cm resolution. Spectra were
measured in CCl
4
solvent at a concentration of ∼0.34 M and
at path lengths of 60 and 130 µm. Spectra for (-)-tert-
butylphenylphosphinothioic acid have also been collected in
CCl
M. The sample was held in a variable path length cell with
BaF windows. In the presented absorption spectra, the solvent
4 3
solvent at different concentrations and in CHCl at 0.38
2
and 3. As can be seen in Figures 2 and 3, the overall
2
spectral patterns did not change significantly in these
calculations. The frequencies of bands 23, 25, and 26 are
predicted to be slightly lower, by 6-31+G and 6-311G-
absorption was subtracted out. In the presented VCD spectra,
the raw VCD spectrum of the solvent was subtracted.
Ca lcu la tion s. The ab initio vibrational frequencies, absorp-
tion, and VCD intensities for (S)-tert-butylphenylphosphi-
(3df, 3pd) basis sets, approaching closer to the experi-
1
7
mental frequencies. But the relative absorption intensity
for band 26 is predicted to be larger than that observed,
and the relative VCD intensity for band 26 is predicted
to be much smaller than that observed. To summarize
these observations, the quality of predicted spectra did
not improve upon going from 6-31G* to the 6-31+G,
nothioic acid were calculated using the Gaussian 98 program
on a Pentium II 300 MHz PC. The calculations used the
density functional theory (DFT) with Becke-style B3LYP and
B3PW9115 hybrid functionals and 6-31G*, 6-31+G, 6-311G-
20
(
2d,2p), and 6-311gG(3df,3pd) basis sets. The procedure for
calculating the VCD intensities using DFT theory from
1
6
Cheeseman et al. as implemented in the Gaussian 98
program.17 The theoretical absorption and VCD spectra were
6
-311G(2d, 2p), or 6-311G(3df, 3pd) basis set. Similarly,
-
1
simulated with Lorentzian band shapes and 5 cm full width
at half-height. Since the ab initio-predicted band positions are
higher than the experimental values, the ab initio frequencies
obtained with 6-31G* basis set were scaled with a factor of
there is not a significant change in the predictions when
the functional is changed from B3LYP/6-31G* to B3PW91/
6
ence between observed and calculated frequencies is
rather large in the 1200-900 cm
-31G*. Finally, it is somewhat unusual that the differ-
0
.9613, and those based on other basis sets are scaled with a
-
1
region for this
factor of 0.975.
molecule. This discrepancy may be attributed to the
presence of heavy atoms in this molecule.
(21) Drabowicz, J .; Mikołajczyk, M. Unpublished results.

Enantiomeric tert-Butylphenylphosphinothioic Acid
Ack n ow led gm en t . Grants from the NSF (CHE-
J . Org. Chem., Vol. 66, No. 26, 2001 9019
butylphenylphosphinothioic acid; a table of calculated frequen-
cies, absorption, and VCD intensities with five different basis
sets; and a table containing the comparison of calculated and
observed frequencies along with vibrational assignments in
0
092922) and Vanderbilt University are gratefully
acknowledged. Studies in Ł o´ d z´ were supported by the
State Committee for Scientific Research (KBN) (Grant
T09A 077 14 to J .D.).
-
1
the 1500-850 cm region. This material is available free of
charge via the Internet at http://pubs.acs.org.
Su p p or tin g In for m a tion Ava ila ble: Cartesian coordi-
nates for the two optimized tautomeric structures of (S)-tert-
J O0107406