Preparation and properties of two novel selenoacetic acids: HCF 2C(O)SeH and ClCF2C(O)SeH

Article abstract of DOI:10.1021/ic202572n

The novel selenocarboxylic Se-acids, HCF₂C(O)SeH and ClCF ₂C(O)SeH, were prepared by treating the corresponding carboxylic acids with Woollins' reagent. The boiling points were extrapolated from the vapor pressure curves to be 364 and 359 K for HCF₂C(O)SeH and ClCF₂C(O)SeH, respectively. Both compounds are unstable at ambient temperatures and decompose to the corresponding seleno anhydrides and release of H2Se. Hydrolysis results in formation of the carboxylic acids and hydrogen selenide, while diselenides presumably are obtained by oxidation. The conformational properties of these acids were studied by vibrational spectroscopy in combination with ab initio and DFT methods. IR vapor-phase spectra, Raman spectra of the neat liquids, and IR spectra of the Ar-matrix-isolated compounds deposited at two different nozzle temperatures were interpreted in terms of quenching conformational equilibria. The most stable structure of both acids was found to be syn-gauche in equilibrium with a second anti-syn form in HCF₂C(O)SeH and with another two conformers, anti-gauche and anti-syn, in ClCF₂C(O)SeH.

Full text of DOI:10.1021/ic202572n

Article
pubs.acs.org/IC
Preparation and Properties of Two Novel Selenoacetic Acids:
HCF C(O)SeH and ClCF C(O)SeH
2
2
†
,⊥
,†
‡
‡
Jovanny A. Gom
and Carlos O. Della Ved
́
ez Castan
̃
o, Rosana M. Romano,* Helmut Beckers, Helge Willner,
†
,§
́
ova
†CEQUINOR (UNLP-CONICET), Departamento de Química, Facultad de Ciencias Exactas, Universidad Nacional de La Plata,
4
7 esq. 115, (1900) La Plata, Argentina
‡
̈
Anorganische Chemie, Bergische Universitat Wuppertal, Gaussstrasse 20, D-42097 Wuppertal, Germany
Laboratorio de Servicios a la Industria y al Sistema Científico (UNLP-CIC-CONICET), Departamento de Química,
Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Camino Centenario e/505 y 508, (1897) Gonnet, Argentina
§
*
S Supporting Information
ABSTRACT: The novel selenocarboxylic Se-acids, HCF C(O)SeH
2
and ClCF C(O)SeH, were prepared by treating the corresponding
2
carboxylic acids with Woollins’ reagent. The boiling points were extra-
polated from the vapor pressure curves to be 364 and 359 K for
HCF C(O)SeH and ClCF C(O)SeH, respectively. Both compounds
2
2
are unstable at ambient temperatures and decompose to the corre-
sponding seleno anhydrides and release of H Se. Hydrolysis results
2
in formation of the carboxylic acids and hydrogen selenide, while
diselenides presumably are obtained by oxidation. The conformational properties of these acids were studied by vibrational
spectroscopy in combination with ab initio and DFT methods. IR vapor-phase spectra, Raman spectra of the neat liquids, and IR
spectra of the Ar-matrix-isolated compounds deposited at two different nozzle temperatures were interpreted in terms of
quenching conformational equilibria. The most stable structure of both acids was found to be syn-gauche in equilibrium with a
second anti-syn form in HCF C(O)SeH and with another two conformers, anti-gauche and anti-syn, in ClCF C(O)SeH.
2
2
INTRODUCTION
Recently, we succeeded in the preparation of small seleno-
carboxylic Se-acids, like selenoacetic Se-acid in its normal and
perdeutereted forms, CH C(O)SeH and CD C(O)SeD, and
the trifluoroselenoacetic Se-acid, CF C(O)SeH. An important
role of these molecules in chemical synthesis, particularly in
In this paper we report the preparation of two hitherto un-
■
known selenocarboxylic Se-acids, HCF C(O)SeH and ClCF C-
2
2
(
O)SeH, by reaction of the corresponding carboxylic acids
1
with Woollins’ reagent, Ph P Se . The vapor pressure curves
2
2
4
3
3
2
and extrapolated boiling points of both compounds were deter-
mined. Vibrational properties were studied by IR vapor-phase
spectra, Raman spectra of the neat liquids, and IR spectra of
the Ar-matrix-isolated compounds deposited at two different
nozzle temperatures. Experimental results were interpreted in
terms of conformational equilibria aided by ab initio and DFT
calculations.
3
reactions involving CX C(O)Se− group transfer, may be antici-
3
pated by comparison with reported applications of the sulfur
3
−5
derivatives.
1
,2
6,7
CX C(O)SeH molecules, like CX C(O)OH and CX C-
O)SH
3
3
3
8
−10
(
compounds, present a preferred syn conformation
(
the CO double bond syn with respect to the Se−H single
EXPERIMENTAL SECTION
bond) in equilibrium with the anti form. If one of the X atoms
of the CX group is replaced by another Y atom, the confor-
mational possibilities are increased, depending on the orienta-
■
Chemicals. Commercially available difluoroacetic acid and chloro-
difluoroacetic acid were purified by repeated trap-to-trap condensation
in vacuum. Woollins’ reagent, Ph P Se , was prepared according the
3
2
2
4
tion of the C−Y bond with respect to the CO group. Several
18
reported procedure.
examples of molecules with general formula YCX C(O)OH
2
Preparation. Difluoroselenoacetic Se-acid, HCF C(O)SeH, was
2
and YCX C(O)SH reported in the literature occur as an equi-
librium mixture of two or more rotamers.
2
prepared by treating HCF C(O)OH with an excess of Woollins’ re-
2
11−17
1
8−20
agent
at 70 °C in a closed glass vessel. The progress of the reac-
Vibrational spectroscopy has been extensively used as a
powerful tool to probe their structure, particularly when com-
bined with theoretical studies. IR spectra of compounds iso-
lated in matrices after passing the spray-on nozzle at different
temperatures usually contain valuable information about the
conformational equilibrium, even for molecules possessing more
than two forms in equilibrium.
tion was followed by vapor-phase IR spectroscopy. After 10 h the
components of the reaction mixture were separated by repeated trap-
to-trap fractional condensation in vacuum. A pure sample of the com-
pound was retained in the −110 °C trap. Starting from about 11 mmol
Received: November 29, 2011
Published: February 3, 2012
©
2012 American Chemical Society
2608
dx.doi.org/10.1021/ic202572n | Inorg. Chem. 2012, 51, 2608−2615

Inorganic Chemistry
Article
of HCF C(O)OH, approximately 0.6 mmol of pure HCF C(O)SeH
2
2
was obtained. For spectroscopical studies the products of several pre-
parations were collected together.
Chlorodifluoroselenoacetic acid, ClCF C(O)SeH, was prepared in
2
the same way by treating the corresponding carboxylic acid, ClCF C-
2
(
O)OH, with Woollins’ reagent. In this case the reaction was
interrupted after 19 h and a pure sample of the compound retained
in a trap held at −100 °C. Starting from about 8.0 mmol of
ClCF C(O)OH, approximately 0.1 mmol of pure ClCF C(O)SeH
2
2
was obtained.
Vapor Pressure. The vapor pressure of the liquid samples was
measured in a small vacuum line equipped with a calibrated capa-
citance manometer (MKS Baratron, AHS-, AHS-100) and a sample
reservoir.
NMR Spectroscopy. For the H, 19F, and 13C NMR measurements
1
neat samples were held in flame-sealed, thin-walled 4 mm tubes which
were placed inside 5 mm NMR tubes. CD OD was used as external
3
lock and TMS and CFCl as reference. Spectra were recorded with a
3
Bruker Avance 250 spectrometer operating at 250.13, 235.36, and
1
19
13
6
2.90 MHz for H, F, and C spectra, respectively. Samples were
maintained at −30 °C during the measurements.
Figure 1. Gas-phase FTIR spectrum of HCF
2
C(O)SeH recorded at
−1
UV−Vis Spectroscopy. UV−vis spectra of gaseous samples were
recorded using a glass cell (optical path length 10 cm) equipped with
quartz windows and placed in the sample compartment of a Lambda
4.7 mbar, 2 cm resolution and an optical path length of 20 cm (top),
and Raman spectrum of the liquid recorded at room temperature and
using λ = 1064 nm excitation and 150 mW (bottom).
9
00 spectrometer (Perkin-Elmer, Norwalk, CT). Measurements were
carried out in the spectral range 200−600 nm.
Vibrational Spectroscopy. IR spectra of the vapor samples were
−1
−1
recorded at a resolution of 2 cm in the range 4000−400 cm with
a Bruker Vector 25 spectrometer. Raman spectra were measured
on neat liquids sealed in 4 mm glass tubes using a Bruker-Equinox
5
5 FRA 106/S FT-Raman spectrometer and a 1064 nm Nd:YAG laser
(
150 mW).
Matrix Measurements. A few milligrams of each of the pure
compounds were transferred into a small U trap connected to the
−
1
inlet nozzle of the matrix apparatus. A gas stream of Ar (2 mmol h )
was directed over the sample held at 158 K, and the resulting gas
mixture at room temperature was condensed onto a rhodium-plated
copper mirror held at 15 K. For experiments at higher temperatures
of the gas mixture the nozzle (i.d. 4 mm, length 20 mm quartz tube
with an end orifice of 1 mm) was heated to 573 (HCF C(O)SeH)
2
or 483 K (ClCF C(O)SeH). Details of the matrix apparatus are
2
2
1
given elsewhere. IR spectra of the Ar matrices were recorded in the
reflectance mode by means of a transfer optic using the Bruker IFS
6
6v spectrometer. An MCT-600 detector, together with a KBr/Ge
−
1
beam splitter, was used in the region 5000−650 cm . One hundred
scans were added for spectra with apodized resolutions of 0.5 and
0
−
1
.15 cm .
Theoretical Calculations. All quantum chemical calculations were
Figure 2. FTIR spectrum of HCF C(O)SeH isolated in solid Ar
(bottom) and calculated with the B3LYP/6-311++G** level of
approximation (top).
2
22
performed with the Gaussian 03 program package. Second-order
Moller−Plesset (MP2) and density-functional (DFT) B3LYP methods
were employed using the 6-311++G** basis set. Geometries were
optimized by standard gradient techniques with simultaneous
relaxation of all geometric parameters.
Exposure of the samples to air results in decomposition of the
compounds, leading apparently to the corresponding diselenide
compounds. CH C(O)SeSeC(O)CH was identified through
3
3
RESULTS AND DISCUSSION
■
its IR and Raman spectra when the selenoacetic Se-acid, CH
(O)SeH, was treated in the same way. Hydrolysis of HCF C-
3
C-
2
23
Difluoroselenoacetic Se-acid, HCF C(O)SeH, melts at 148
2
(
2) K, and the vapor pressure curve in the temperature range
(O)SeH and ClCF C(O)SeH yields the carboxylic acids and
2
2
14−290 K follows the equation ln p [atm] = 11.31 − 4117/
hydrogen selenide, as characterized by their IR spectra in the
T [K], giving an extrapolated boiling point of 364 K (see
Figures S1 and S2 in the Supporting Information). The vapor
gas phase.
1
NMR Spectroscopy. The H NMR spectrum of neat
pressure curve of chlorodifluoroselenoacetic Se-acid, ClCF C-
ClCF C(O)SeH shows a singlet at 2.8 ppm, which is close to
2
2
2
(
O)SeH, follows the equation ln p [atm] = 12.51 − 4494/T
the reported chemical shift for CF C(O)SeH (3.1 ppm), and
3
[
K] in the temperature range 236−290 K, giving an extra-
in the range previously reported for related selenocarboxylic Se-
24
19
polated boiling point of 359 K (see Figures S3 and S4 in the
Supporting Information).
acids RC(O)SeH. The F NMR spectrum reveals a singlet at
−64.8 ppm, which has been assigned to the two fluorine atoms
Both compounds are unstable at ambient temperatures in
sealed glass vessels, giving an orange solid (presumably seleno-
of the ClCF − group. This value is 13.8 ppm shifted to higher
2
frequency with respect to the observed signal for CF C(O)-
3
2
anhydride) and H Se, detected by its gas-phase IR spectrum.
SeH, occurring at −78.6 ppm.
2
2
609
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Inorganic Chemistry
Article
−1
Table 1. Experimental and Calculated Vibrational Wavenumbers (in cm ) of HCF C(O)SeH (relative intensities are given in
2
parentheses)
a
experimental
liquid Raman
MP2/6-311++G**
b
vapor IR
Ar matrix
syn-gauche
anti-syn
assignment
3
468 (2)
2ν(CO) s-g
ν(C−H) a-s
ν(C−H) s-g
ν(Se−H) a-s
ν(Se−H) s-g
ν(CO) s-g
2
997.0 (8)
2994.0 (10)
2316.2 (<1)
2301.9 (<1)
3180.2 (5)
2540.9 (2)
2
2
2
1
988 (3)
2992 (33)
3175.9 (7)
343 (<1)
333 (<1)
747 (100)
2323 (100)
1719 (15)
2533.3 (3)
1735.9⎫
1744.1 (100)
⎪
⎪
1
1
734.0⎬ (100)
730.8⎭
1
679 (6)
365 (3)
1724.8 (21)
1738.7 (100)
1385.3 (4)
ν(CO) a-s
1
718.8 (5)
ν(CO) dimer?
δ(HCF) dimer?
1
1
355.2⎫
⎬
(1)
351.1⎭
1
1346.6 (6)
1392.3 (9)
δ(HCF) s-g
δ(HCF) a-s
1
1
345.4⎫
⎬
(4)
344.5⎭
1
342.8 (2)
δ(HCC) dimer?
δ(HCC) s-g
1
341 (24)
1341 (11)
1152 (9)
1335.3 (22)
1381.9 (14)
1
1
333.2⎫
⎬
1372.0 (16)
1198.5 (31)
δ(HCC) a-s
(4)
330.8⎭
1
163.1 (3)
ν(C−C) a-s
1
155 (34)
096 (52)
1159.9 (32)
1197.5 (40)
1126.7 (57)
ν(C−C) s-g
1
1
154.8⎫
⎬
ν(C−C) dimer?
(19)
148.1⎭
1
1103.3 (73)
ν (CF ) s-g
as
2
1
1
097.6⎫
1113.9 (11)
1108.0 (75)
ν
s
(CF ) a-s
2
⎬
(16)
(27)
(29)
094.6⎭
1
1
087.4⎫
νas(CF ) a-s
2
⎬
081.2⎭
1075 (25)
1061.0⎫
1093.0 (50)
ν (CF ) s-g
s 2
⎬
1
7
7
059.3⎭
97.4 (1)
86.7 (7)
δ(HSeC) dimer?
δ(HSeC) a-s
δ(HSeC) s-g
δoop(CO) s-g
δoop(CO) a-s
δ(OCC) a-s
δ(OCC) s-g
νs(C−Se) a-s
νs(C−Se) s-g
δ(CF2)
855.7 (2)
795 (6)
712 (6)
678 (4)
607 (1)
594 (2)
797 (12)
603 (39)
785.6 (4)
852.9 (3)
687.6 (13)
685.3 (5)
610.3 (<1)
602.5 (6)
692.1 (19)
662.9 (<1)
624.2 (2)
610.7 (2)
5
50.9 (8)
562.0 (34)
541 (12)
546.8 (4)
520.0 (34)
466.5 (2)
351.8 (<1)
4
40 (15)
49 (37)
446.7(2)
3
288.6 (<1)
339.0 (<1)
207.9 (2)
166.7 (1)
28.9 (2)
δ(CF2)
329.2 (5)
234.6 (1)
162.6 (1)
δ(SeCO)
δ(CF2)twist
δ(SeCC)
3
1.2 (1)
τ
a
b
Relative intensities between parentheses. s-g = syn-gauche, a-s = anti-syn.
The 13C NMR spectrum has two triplets centered at
ClCF − and CO groups, respectively. These results
2
1
2
1
19.4 ( J(
= 305.7 Hz) and 185.8 ( J
= 37.7 Hz)
are consistent with reported values for related mole-
C−F)
(C−C−F)
2
5,26
ppm, which were assigned to the carbon atom of the
cules.
2
610
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Figure 5. B3LYP/6-311++G** molecular models for the syn-gauche
(
left) and anti-syn (right) conformers of HCF C(O)SeH.
2
Figure 6. B3LYP/6-311++G** molecular models for the syn-gauche
left), anti-gauche (middle), and anti-syn (right) conformers of
(
ClCF C(O)SeH.
2
predicted spectra, (iii) relative band intensity changes observed
in the matrix spectra at different nozzle temperatures, and (iv)
the photochemical behavior of the bands.
Figure 3. Gas-phase FTIR spectrum of ClCF C(O)SeH recorded at
2
−
1
9
.3 mbar, 2 cm resolution and an optical path length of 20 cm (top),
and Raman spectrum of the liquid at room temperature and using λ =
064 nm excitation and 150 mW (bottom).
27
1
The vapor-phase IR and Raman spectra of liquid HCF C-
2
(
O)SeH are presented in Figure 1, while Figure 2 shows the
Ar-matrix IR spectrum taken at ambient temperature of the spray-
on nozzle and the one simulated by DFT calculations. Table 1
compiles the experimental wavenumbers together with the cal-
culated ones obtained by ab initio methods (see the Theoretical
Calculations section) and presents a tentative assignment.
Spectra were assigned by comparison with theoretical cal-
culations, which predict the most stable syn-gauche con-
former in equilibrium with an anti-syn form (see the Theoretical
Calculations section). Fourteen (12) of the 18 vibrational normal
modes of the most (less) stable rotamer were observed.
The C−H stretching fundamental is clearly observed at
992 cm− in the Raman spectrum and at 2988 cm in the
1
−1
2
vapor-IR spectrum as a weak absorption. However, two bands
−1
are discernible in the matrix spectra at 2997.0 and 2994.0 cm .
Taking into account the increase of the relative intensity of the
997.0 cm− absorption at the expense of the 2994.0 cm IR
1
−1
2
matrix feature when the temperature of the spray-on nozzle
was raised from ambient temperature to 583 K and the +4 cm−
shift predicted by the MP2/6-311++G** method for the
second form with respect to the most stable one, these two
absorptions were assigned to the anti-syn and syn-gauche
conformer, respectively.
1
Figure 4. FTIR spectrum of ClCF C(O)SeH isolated in solid Ar
2
(
bottom) and calculated with the B3LYP/6-311++G** level of
approximation (top).
The Raman spectrum of the liquid sample is dominated by a
strong band centered at 2323 cm , attributed to the ν Se−H
fundamental mode. A splitting of this band in the IR spectra
vapor-phase, Ar matrix) indicates the presence of two con-
formers.
The most intense absorption in the vapor-phase IR spectrum
UV−Vis Spectroscopy. Vapor-phase UV−vis spectra of
both selenoacetic Se-acids were recorded in the range from
−1
2
00 to 600 nm. A broad absorption centered at 252 and
(
253 nm for HCF C(O)SeH and ClCF C(O)SeH, respectively,
2
2
was attributed to the lp(π)Se → π*
transition (spectra are
presented in Figures S5 and S6, Supporting Information).
CO
corresponds to the ν CO vibrational mode, occurring at
These values are very close to the 251 nm UV absorption
2
−1
1
747 cm as a unique peak. In the Raman spectrum two bands
reported for trifluoroselenoacetic Se-acid, indicating similar
inductive effects of the CF , HCF , and ClCF groups on the
π lone pair orbital located at the Se atom.
are detectable in this spectral region. The matrix spectra taken
after passing the gas mixture to a nozzle at different tem-
peratures also proved the presence of two conformers, as pre-
sented in Table 1.
3
2
2
Vibrational Spectroscopy. The vibrational properties of
both species were studied through the IR spectra of the vapor
phase, Raman spectra of the liquid samples, and IR spectra of
Ar-matrix-isolated compounds deposited at two different nozzle
temperatures. Spectra were interpreted and assigned based
on (i) comparison with related molecules, (ii) theoretically
A sharp peak with medium intensity appears at 1341 cm− in
the IR spectrum of the vapor and at the same position in the
Raman spectrum of the liquid and was assigned to δ(HCC)
of the syn-gauche form. The bands corresponding to both
1
2
611
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Inorganic Chemistry
Article
−1
Table 2. Experimental and Calculated Vibrational Wavenumbers (in cm ) of ClCF C(O)SeH (relative intensities are given in
2
parentheses)
a
experimental
liquid Raman
MP2/6-311++G**
b
vapor IR
Ar matrix
syn-gauche
2535.6 (2)
anti-gauche
anti-syn
assignment
3
492 (1)
340 (1)
2ν(CO) s-g
2
2329 (100)
1731 (13)
2551.2 (2)
2542.9 (3)
ν(Se−H)
1
1
748.2⎫
1757.4 (100)
ν(CO) a-s
⎬
(42)
745.7⎭
1
759 (100)
754 (40)
1740.1 (93)
1737.7 (44)
1757.1 (100)
1240.7 (33)
ν(CO) s-g
ν(CO) a-g
ν(C−C) a-g
ν(C−C) s-g
ν(C−C) a-s
ν (CF ) s-g
1
1752.7 (100)
1235.5 (31)
1
216.2 (15)
1214.1 (30)
211.0 (24)
1158.8 (98)
1
208 (27)
160 (64)
1202 (4)
1150 (2)
1
1197.7 (38)
1
1179.2 (74)
as
2
1
1
156.3 (53)
152.6 (26)
1174.2 (55)
ν (CF ) a-g
as 2
1121.4 (90)
1107.6 (46)
ν (CF ) a-s
as 2
1
055 (12)
016 (46)
1056 (3)
1011 (5)
1033.8⎫
ν (CF ) a-s
s 2
⎪
⎪
1
1
031.6⎬ (17)
028.7⎭
1
1014.1⎫
1052.5 (96)
ν (CF ) s-g
s
2
⎬
(56)
1
013.2⎭
1
003.5⎫
1041.4 (83)
906.0 (51)
ν
s
(CF
) a-g
2
⎪
9
98.8 ⎬ (22)
⎪
9
96.0 ⎭
8
8
8
77.1⎫
δ(CF )wag a-g
2
⎪
75.1⎬ (10)
⎪
73.5⎭
8
8
7
73 (29)
53 (75)
62 (7)
847 (5)
765 (8)
856.3 (100)
843.1 (81)
766.7⎫
899.3 (69)
786.1 (49)
δ(CF2)wag s-g
δ(CF2)wag a-s
δ(HSeC) a-g
892.8 (45)
752.6 (81)
804.2 (33)
⎬
(4)
7
62.4⎭
7
7
43 (24)
23 (14)
748.4⎫
δ(HSeC) s-g
δ(HSeC) a-s
⎪
7
7
47.0⎬ (19)
⎪
40.0⎭
728.3⎫
⎪
7
7
23.8⎬ (6)
⎪
20.5⎭
6
63.6 (6)
674.3 (2)
δoop(CO) a-s
δoop(CO) s-g
δoop(CO) a-g
ν(C−Cl) a-s
ν(C−Cl) s-g
6
65 (9)
88 (4)
667 (41)
592 (12)
661.0 (3)
673.6 (8)
599.9 (4)
6
57.2 (1)
87.5 (5)
672.5 (6)
5
668.8 (<1)
5
585.7⎫
⎬
(11)
5
83.8⎭
5
81.5 (3)
601.7 (4)
494.3 (3)
437.0 (<1)
ν(C−Cl) a-g
δ(OCSe)
ν(C−Se)
δ (CF )
4
88 (4)
25 (1)
490 (13)
427 (22)
508.7 (6)
438.9 (1)
389.1 (1)
348.1 (<1)
334.0 (4)
280.1 (2)
185.3 (<1)
160.7 (<1)
463.6 (2)
443.4 (2)
4
384 (30)
373 (16)
343 (19)
280 (15)
190 (12)
156 (10)
s
2
288.6 (1)
348.6 (<1)
276.6 (2)
187.7 (2)
158.7 (1)
40.4 (<1)
349.0 (1)
290.8 (1)
227.1 (<1)
215.6 (3)
162.2 (2)
27.6 (1)
τ(HSeCO)
δ(OCC)
δ (CF )
s
2 twist
δ(ClCC)
δ(SeCC)
τ
3
3.5 (<1)
a
b
Relative intensities between parentheses. s-g = syn-gauche, a-s = anti-syn.
conformers are observed in the spectrum of the matrix-isolated
species, as predicted by ab initio calculations.
comparison with the IR spectra of related molecules (see, for
example, ref 28) and taking into account the prediction of
theoretical calculations. As expected, a more complex pattern
was observed in this spectral region in the IR spectra of matrix-
−1
Three absorptions, at 1155, 1096, and 1075 cm , appear in
−1
the 1200−1000 cm region of the IR spectrum. These bands
were assigned to νC−C, ν CF , and ν CF , respectively, by
isolated HCF C(O)SeH. Not only the normal modes of the
as
2
s
2
2
2
612
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Scheme 1. Possible Conformations of Molecules with the General Formula YCX C(O)EH
2
Table 3. Experimental and Calculated Molar Ratios and
Theoretically Predicted Relative Energies and Free
Energies of the syn-gauche and anti-syn Rotamers of
In addition to the absorptions assigned to the syn-gauche and
anti-syn forms of HCF C(O)SeH, some low-intensity features
2
observed in the IR matrix spectra that decrease their intensity
with the increase of the nozzle temperature were tentatively
assigned to a hydrogen-bonded dimer. The wavenumbers and
the proposed assignment were included in Table 1.
HCF C(O)SeH
2
2
98 K
573 K
a
a
ΔE
s-g % a-s % s-g % a-s [kJ/mol]
ΔG°
The IR vapor-phase spectrum of ClCF C(O)SeH is shown in
2
%
[kJ/mol]
Figure 3, together with the Raman spectrum of the neat liquid.
b
Ar matrix
MP2/6-311+
G**
B3LYP/6-311+
G**
84.5
15.5
31.1
83.6
67.8
16.4
32.2
Figure 4 depicts the FTIR spectrum of ClCF C(O)SeH iso-
2
68.9
0.25
0.29
0.25
0.75
lated in solid Ar and the computed spectrum at the B3LYP/
-311++G** level of approximation for the theoretically pre-
dicted most stable conformer (see the Theoretical Calculations
section). Table 2 compiles the experimental wavenumbers of
the three spectra, the predicted vibrational frequencies of the
different conformers, and a tentative assignment of the bands.
The criteria used for the assignment of the bands were the same
+
6
73.0
27.0
70.1
29.9
+
a
Relative energy of the anti-syn (a-s) rotamers with respect to the
b
most stable syn-gauche (s-g) including ZPE corrections. Experi-
mental values are obtained from integrated intensities of the ν(CO)
absorptions in the Ar-matrix IR spectra corrected by calculated
absorption coefficients obtained at the B3LYP/6-311++G** level.
as those described for vibrational analysis of HCF C(O)SeH
2
(
vide infra).
The vapor-phase IR spectrum of ClCF C(O)SeH is domi-
2
most stable conformer but also the ones attributable to the
anti-syn form are present in the spectra. According to MP2 predic-
tion, the order of the symmetric and antisymmetric stretch-
nated by two strong absorptions in the carbonylic stretching
−1
region at 1759 and 1754 cm , which were assigned to the syn-
gauche and anti-gauche conformers, respectively, according to
the MP2 prediction. As can be seen in Table 2, several bands of
the vapor-phase IR were attributed to the second anti-gauche
form.
ing of the CF group is inverted for the second conformer
2
with respect to the most stable one (see Table 1). Only the νC−C
−1
mode, occurring at 1152 cm , is observed in the Raman spectrum
in this region.
The FTIR spectra of the Ar-matrix-isolated compound which
pass the spray-on nozzle at ambient temperature and also at
83 K before matrix deposition confirm the presence of three
conformers in equilibrium. Eight fundamentals of each of the
The νC−Se fundamental was observed only in the IR
−1
spectra, appearing at 541 cm in the vapor-phase spectrum and
4
−1
at 546.8 (syn-gauche) and 550.9 (anti-syn) cm in the IR
matrix spectra. Bands at lower wavenumbers were assigned to
deformation and torsional modes as described in Table 1.
three conformers of ClCF C(O)SeH were observed. A com-
2
plete description of the IR spectra is presented in Table 2.
Table 4. Experimental and Calculated Molar Ratios and Relative Energies of the syn-gauche (s-g), anti-gauche (a-g) and anti-
syn (a-s) Rotamers of ClCF C(O)SeH
2
a
a
2
98 K
483 K
% a-g
ΔE [kJ/mol]
ΔG° [kJ/mol]
%
s-g
% a-g
% a-s
% s-g
% a-s
s-g
a-g
a-s
s-g
a-g
a-s
b
Ar matrix
69.1
78.8
72.6
22.0
15.6
18.2
8.9
5.6
9.2
56.3
63.2
61.0
29.0
23.2
25.9
14.7
13.6
13.1
MP2/6-311++G**
B3LYP/6-311++G**
0.00
0.00
3.85
3.27
5.44
4.98
0.00
4.02
3.43
3.39
3.39
0.00
a
b
Relative energy of the less stable rotamers with respect to the most stable syn-gauche (s-g) including ZPE corrections. Experimental values are
obtained from integrated intensities of the ν(CO) absorptions in the Ar-matrix IR spectra corrected by calculated absorption coefficients obtained
at the B3LYP/6-311++G** level.
2
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The most distinct feature in the Raman spectrum of
(see Figure 6). Table S2, Supporting Information, lists the
calculated geometrical parameters, and Table 4 presents calcu-
lated relative energies and standard Gibbs free energy diffe-
rences, together with the Boltzmann distribution at ambient
temperature, for the three forms. These theoretical findings
are in complete agreement with the conformational behavior
−1
ClCF C(O)SeH corresponds to the SeH band at 2329 cm .
2
In addition, the Raman spectrum provides the vibrational
information of the low-energy region, not studied in the IR1
spectra. Thus, bands at 384, 373, 343, 280, 190, and 156 cm−
were assigned to the δ (CF ), τ(HSeCO), δ(OCC),
s
2
δs(CF2)twist, δ(ClCC), and δ(SeCC), respectively.
Theoretical Calculations. Theoretical conformational
reported for ClCF C(O)SH based on vibrational analysis and
theoretical calculations.
2
15
studies of compounds of the general formula RC(O)EH, with
Theoretical vibrational spectra of each of the stable con-
formers of HCF C(O)SeH and ClCF C(O)SeH were com-
R = CX and E = O, S, or Se, predict the most stable syn
3
2
2
conformation (the CO group syn with respect to the E−H
single bond) in equilibrium with the anti form. These results
are in agreement with the experimental finding for several
puted at the B3LYP/6-311++G** and MP2/6-311++G** level
to characterize them as true minima and to help in the assign-
ment of the experimental IR and Raman spectra. The calculated
6
8
derivatives of this family: CH C(O)OH, CH C(O)SH,
wavenumbers for the different conformers of HCF
are presented in Table 1, while the corresponding data of
ClCF C(O)SeH are compiled in Table 2.
C(O)SeH
2
3
3
1
7
9,10
CH C(O)SeH, CF C(O)OH, CF C(O)SH,
and CF C-
3
3
3
3
3
2
(
O)SeH. If one of the X atoms of the −CX group is
2
substituted for another atom Y, the conformational possibilities
are increased, and the orientation of the CO group with
respect to the E−H (syn or anti) and the C−Y single bond
CONCLUSIONS
Two hitherto unknown compounds of the selenocarboxylic Se-
acid family, difluoroselenoacetic Se-acid, HCF C(O)SeH, and
■
(
syn, gauche, or anti) should be considered, as depicted in the
2
Scheme 1.
chlorodifluoroselenoacetic Se-acid, ClCF C(O)SeH, were pre-
2
11−14
Compounds such as chloroacetic acid, H CClC(O)OH,
and chlorodifluorothioacetic S-acid, ClCF C(O)SH, present
pared by an O/Se exchange reaction using Woollins’ reagent
and the corresponding carboxylic acid. The physical properties
of these two compounds (boiling points and vapor pressure
curves) and chemical reactions at ambient temperatures with
oxygen and water were studied. The compounds were investi-
gated by vibrational (vapor-phase IR, liquid Raman, and matrix-
isolated IR) and UV−vis spectroscopies.
The conformational preferences of these acids were pro-
posed based on interpretation of the vibrational spectra, parti-
cularly the IR spectra of the compounds isolated in solid Ar at
different nozzle temperatures, and with the aid of predictions
using ab initio and DFT methods. The most stable structure of
both acids was found to be the syn-gauche form. The presence
of a second rotamer, with anti-syn conformation, is predicted
2
15
2
an important contribution of the anti form in equilibrium with
the most stable syn conformer. For H CClC(O)OH the anti-
2
gauche conformation is the second stable form (of a total of
11
three) as derived from the gas electron diffraction and micro-
1
2
wave analysis as well as matrix isolation results interpreted in
1
3,14
terms of computational calculations.
On the other hand,
the two most stable structures of ClCF C(O)SH according to
2
theoretical calculations correspond to the syn-gauche and anti-
1
5
gauche forms.
The contribution of the anti form in acids possessing the
difluoromethyl group, −HCF , is also relevant. For example,
2
16
17
for HCF C(O)OH gas electron diffraction and microwave
2
results were interpreted in terms of the presence of two forms
in equilibrium, although both studies differ in the relative pro-
portion of each conformer.
for HCF C(O)SeH and consistent with the experimental find-
2
ing. In the case of chlorodifluoroselenoacetic Se-acid, the anti-
gauche and anti-syn forms were detected in equilibrium with
the most stable syn-gauche one.
In this contribution, the conformational properties of
HCF C(O)SeH and ClCF C(O)SeH acids were theoretically
investigated using the B3LYP/6-311+G* model through a
relaxed bidimensional scan in which the two dihedral angles
2
2
ASSOCIATED CONTENT
Supporting Information
Additional information as noted in the text. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
*
S
OCSeH and HCCO or ClCCO for HCF C(O)SeH and
2
ClCF C(O)SeH, respectively, were simultaneously varied in
2
steps of 10° (Figures S7 and S8, Supporting Information). Only
two structures, namely, syn-gauche and anti-syn, have been
found to be minima at the potential energy surface of the
AUTHOR INFORMATION
Corresponding Author
E-mail: romano@quimica.unlp.edu.ar.
■
*
HCF C(O)SeH molecule. These structures were then fully
2
optimized using the B3LYP/6-311++G** and MP2/6-311+
Present Address
+
G** theoretical approximations. Computed relative energies
⊥
Escuela de Ciencias Quım
Universidad Pedagogica y Tecnolog
Avenida Central del Norte, (150003) Tunja, Boyaca,
́
icas, Facultad de Ciencias,
ica de Colombia (UPTC),
Colombia.
and standard Gibbs free energy differences for the two con-
formers are listed in Table 3, together with the relative abun-
dance of each rotamer at ambient temperature. Figure 5 shows
molecular models of the conformers, and Table S1, Supporting
Information, compiles their calculated geometrical parameters.
These two conformers have also been proposed for difluoro-
́
́
́
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
C.O.D.V. and R.M.R. thank the Consejo Nacional de
acetic acid, HCF C(O)OH, according to gas electron diffrac-
■
2
1
6
tion studies.
Analysis of the potential energy surface calculated for ClCF C-
Investigaciones Cientıf
́
icas y Tec
́
nicas (CONICET) (PIP
Cientıfica y
ica (ANPCyT, PICT 33878 and 322), the Comision
icas de la Provincia de Buenos Aires
2
(
O)SeH reveals the presence of three minima, corresponding
0352), the Agencia Nacional de Promocion
Tecnolog
de Investigaciones Cientıf
́
́
to the syn-gauche, anti-gauche, and anti-syn forms. Full optimi-
zation of these conformers using the B3LYP/6-311++G** and
MP2/6-311++G** models confirms them as stable structures
́
́
́
(CIC), and the Facultad de Ciencias Exactas, UNLP for
2
614
dx.doi.org/10.1021/ic202572n | Inorg. Chem. 2012, 51, 2608−2615

Inorganic Chemistry
Article
financial support. J.A.G.C. acknowledges an award from the
Deutsche Akademische Austauschdienst (DAAD). C.O.D.V.
especially acknowledges the DAAD, which generously
supported the DAAD Regional Program of Chemistry of the
Republic Argentina that encourages Latin-American students to
realize their Ph.D. work in La Plata.
Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03; Revision B.04 ed.;
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(26) Silverstein, R. M; Bassler, G. C.; Morrill, T. C. Spectrometric
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