Anomeric effects in sulfonyl compounds: An experimental and computational study of fluorosulfonyl azide, FSO2N3, and trifluoromethylsulfonyl azide, CF3SO2N3

Article abstract of DOI:10.1021/jp103616q

Fluorosulfonyl azide, FSO₂N₃, was characterized by IR (gas phase, Ar matrix), and Raman (liquid) spectroscopy. According to the matrix IR spectrum of ¹⁸O-labeled FSO₂N₃, its two oxygen atoms are nonequivalent. This assumption was confirmed by the X-ray crystal structure of FSO₂N₃ at -123 °C, as only one conformer was observed with one of the S - O bonds in synperiplanar position to the N₃ group (φ(OS-NN) = -14.8(3)°) with respect to the S-N bond. The same conformation was found for trifluoromethylsulfonyl azide, CF ₃SO₂N₃ (φ(OS-NN) = -23.74(15)°), in the solid state. The preference of such a synperiplanar configuration between S - O and N₃ was rationalized by a predominant anomeric interaction of n_ω(N) → ω * (S-O), as supported by the experiment and quantum chemical calculations.

Full text of DOI:10.1021/jp103616q

7624
J. Phys. Chem. A 2010, 114, 7624–7630
Anomeric Effects in Sulfonyl Compounds: An Experimental and Computational Study of
Fluorosulfonyl Azide, FSO2N3, and Trifluoromethylsulfonyl Azide, CF3SO2N3
Xiaoqing Zeng,† Michael Gerken,*,†,‡ Helmut Beckers,† and Helge Willner*,†
Bergische UniVersita¨t Wuppertal, FB CsAnorganische Chemie, Gauss Strasse 20,
D-42097 Wuppertal, Germany, and Department of Chemistry and Biochemistry,
The UniVersity of Lethbridge, Lethbridge, Alberta T1K 3M4, Canada
ReceiVed: April 22, 2010; ReVised Manuscript ReceiVed: June 9, 2010
Fluorosulfonyl azide, FSO2N3, was characterized by IR (gas phase, Ar matrix), and Raman (liquid) spectroscopy.
According to the matrix IR spectrum of 18O-labeled FSO2N3, its two oxygen atoms are nonequivalent. This
assumption was confirmed by the X-ray crystal structure of FSO2N3 at -123 °C, as only one conformer was
observed with one of the SdO bonds in synperiplanar position to the N3 group (φ(OS-NN) ) -14.8(3)°)
with respect to the S-N bond. The same conformation was found for trifluoromethylsulfonyl azide, CF3SO2N3
(φ(OS-NN) ) -23.74(15)°), in the solid state. The preference of such a synperiplanar configuration between
SdO and N3 was rationalized by a predominant anomeric interaction of nσ(N) f σ*(S-O), as supported by
the experiment and quantum chemical calculations.
Introduction
On the other hand, a GED study suggested the existence of
a second conformer c for ClSO2NCO beside conformation
b.7 However, more recent computational and vibrational
Due to the medical and chemical importance, compounds
containing sulfur-nitrogen bonds are of long-term interest with
a focus on the relationship between structure and reactivity.1
Ofparticularinterestistheconformationaboutthesulfur-nitrogen
bond in such compounds. Recently, extensive studies have been
performed on organic sulfur-nitrogen single-bonded sulfamides
(RSO2NR2).2 These sulfamides were found to adopt an anti-
periplanar configuration of the nitrogen lone-pair (nN) to the
S-C bond, and short S-N bonds were always observed, as
determined by X-ray crystallography and gas electron diffraction
(GED). This conformation was attributed to a dominant ano-
meric interaction of nσ(N) f σ*(S-C), which results in partial
double-bond character between sulfur and nitrogen (Scheme 1).
SCHEME 1: Description of nσ(N) f σ*(S-C) Anomeric
Interaction in Sulfamides
Sulfonyl azides, RSO2N3, which have been widely used
in organic synthesis,3 should, in principle, also exhibit such
nσ(N) f σ*(S-C) anomeric interactions with a conformation
similar to that found for sulfamides due to the availability
of lone-pair electrons at the R-nitrogen atom as depicted in
Scheme 2. Resonance structure I in Scheme 2 is clearly the
dominant resonance structure for covalent azides, having two
distinctly different lone pairs on the R-nitrogen, i.e., a σ-lone
pair and a π-lone pair. Several conformations of sulfonyl
azides are possible (Scheme 3), with the sterically favored
staggered conformations c and d and eclipsed conformations
a and b which require electronic (anomeric) effects for their
stabilization. Vibrational spectroscopic and GED studies4 of
trifluoromethylsulfonyl azide, CF3SO2N3, suggested a prefer-
ence of one single, sterically nonfavorable conformation with
a nearly eclipsed orientation of the N3 group to one SdO
bond (conformation b in Scheme 3) with a dihedral angle of
φ(OS-NN) ) -24(6)° derived from GED.4a Similar studies
of related molecules, such as CF3SO2NCO5 and FSO2NCO,6
also revealed conformation b in both gas and liquid states.
SCHEME 2: Resonance Structures of Sulfonyl Azides
SCHEME 3: Possible Conformations of Sulfonyl Azide,
RSO2N3
* To whom correspondence should be addressed, michael.gerken@uleth.ca
and willner@uni-wuppertal.de.
† Bergische Universita¨t Wuppertal.
‡ University of Lethbridge.
10.1021/jp103616q  2010 American Chemical Society
Published on Web 06/28/2010

Anomeric Effects in Sulfonyl Compounds
J. Phys. Chem. A, Vol. 114, No. 28, 2010 7625
studies of the gas, liquid, and solid phases did not find any
evidence for the existence of a second conformer beside the
dominant conformer b.8
According to these experimental observations, the stabilizing
anomeric interaction, nσ(N) f σ*(S-O), seems to be more
important in determining the conformation of sulfonyl azides
XSO2N3 and isocyanates XSO2NCO than the nσ(N) f σ*(S-C)
or σ*(S-X) (X ) F, Cl) interactions. The determination of their
high-accuracy molecular structures has not been achieved up
to now, partially because these molecules are gaseous or liquid
under ambient conditions and, hence, X-ray crystallography is
challenging. The possible existence of several conformers
(Scheme 3) for sulfonyl azides due to anomeric and steric
interactions stimulated us to continue our spectroscopic, struc-
tural, and computational studies of simple azides.
As the simplest accessible sulfonyl azide, FSO2N3 was first
prepared by Ruff in 1965 and has only been characterized by
gas-phase IR and UV spectroscopy, as well as mass spectrom-
etry.9 A recent computational study of FSO2N3 suggested
conformer b as the only stable conformer.10 However, no
experimental characterization of its structure has been reported.
The related compound CF3SO2N3 has been widely used as a
powerful reagent for preparation of sulfamides and R-nitro-R-
diazocarbonyl derivatives.11 Herein we report the unambiguous
spectroscopic proof for the nonequivalence of the two oxygen
atoms in FSO2N3 using matrix-IR spectroscopy of 15N- and 18O-
labeled samples, which is further confirmed by determining the
molecular structure of FSO2N3 using low-temperature X-ray
crystallography. This X-ray crystallographic and matrix-IR
spectroscopic study is extended to CF3SO2N3, confirming the
previous prediction of its conformation.4 The conformational
properties of both compounds were discussed in conjunction
with results from quantum chemical calculations.
Figure 1. Calculated potential energy curves for internal rotation
around the S-N bonds in FSO2N3 (9) and CF3SO2N3 (O) at the
B3LYP/6-311+G(3df) level of theory.
Computational Details. Geometry optimizations were per-
formed using DFT methods (B3LYP,14 BP86,15 MPW1PW9116)
with 6-311+G(3df) basis set. Natural bond orbital (NBO)
analysis was performed using RHF/6-31G(d) method for the
evaluation of donor-acceptor (lone pair-antibonding orbital)
interaction energies E(2), based on second-order perturbation
theory.17 The Lewis structure I in Scheme 2 was chosen for the
NBO analysis by using the NBO $CHOOSE keyword in the
NBO 5.0 program.18 The 2-D contour plots of vicinal interac-
tions between preorthogonal natural bond orbitals (PNBOs) are
displayed using the program NBOView.19 All the calculations
were performed using the Gaussian 03 software package.20
X-ray Crystal Structure Determination. (a) Crystal Growth
and Crystal Mounting. Crystals of FSO2N3 and CF3SO2N3 were
grown in an L-shaped glass tube (0.6 cm o.d., 20 cm length).
Small amounts (ca. 20 mg) of the compounds were condensed
at -196 °C into the upper part (above the bent) of the tube,
which was connected to the vacuum line and subsequently
flame-sealed. For the crystallization of FSO2N3 and CF3SO2N3,
the end without sample was immersed into an ethanol cold bath
at ca. -80 °C, while the whole setup with cold bath was kept
in a refrigerator at -20 °C overnight. The glass tube containing
the crystals was cut in a cold nitrogen stream (ca. -70 °C) while
maintaining the sample at -196 °C, and the colorless crystals
were quickly transferred into a trough cooled by a flow of cold
nitrogen and mounted as previously described.21 Crystals of
FSO2N3 and CF3SO2N3 having the dimensions 0.290 × 0.097
× 0.065 and 0.123 × 0.096 × 0.024 mm3, respectively, were
selected at ca. -70 °C under the microscope.
(b) Collection and Reduction of X-ray Data. Crystals were
centered on an Oxford Diffraction Gemini E Ultra diffracto-
meter, equipped with a 2K × 2K EOS CCD area detector, a
four-circle kappa goniometer, an Oxford Instruments Cryojet,
and sealed-tube Enhanced (Mo) and the Enhanced Ultra (Cu)
sources. For the data collection the Mo source emitting graphite-
monochromated Mo KR radiation (λ ) 0.71073 Å) was used.
The diffractometer was controlled by the CrysAlisPro graphical
user interface (GUI) software.22 Diffraction data collection
strategies for FSO2N3 and CF3SO2N3 were optimized with
respect to complete coverage and consisted of four and three ω
scans with a width of 1°, respectively. The data collections for
FSO2N3 and CF3SO2N3 were carried out at -123 °C in a 1024
× 1024 pixel mode using 2 × 2 pixel binning. Processing of
the raw data, scaling of diffraction data, and the application of
Experimental Section
Caution! CoValent azides are potentially explosiVe, although
no explosions were experienced during this work, appropriate
safety precautions must be taken.
Syntheses of FSO2N3 and CF3SO2N3. Fluorosulfonyl azide,
FSO2N3, was prepared by the reaction of (FSO2)2O and NaN3
according to literature.9 For the preparation of 15N-labeled
FSO2N3, 1-15N sodium azide (98 atom % 15N, EURISO-TOP
GmbH) was used. For the preparation of 18O-labeled FSO2N3,
18O-labeled F2S2O5 was used, which was prepared from the
reduction of 18O-labeled (FSO3)2 (60 atom % 18O)12 with CO
in the gas phase at room temperature. Trifluoromethylsulfonyl
azide, CF3SO2N3, was prepared by the reaction of (CF3SO2)2O
and NaN3 according to literature.9 For the preparation of 15N-
labeled CF3SO2N3, 1-15N sodium azide (98 atom % 15N,
EURISO-TOP GmbH) was used. The quality of the samples
was ascertained by gas-phase IR spectroscopy.
Preparation of the Matrices. Matrix IR spectra were
recorded on a FT-IR spectrometer (IFS 66v/S Bruker) in
reflectance mode using a transfer optic. A KBr beam splitter
and an MCT detector were used in the region of 5000-550
cm-1. A Ge-coated 6 µm Mylar beam splitter combined with a
He(l)-cooled Si bolometer and a CsI window at the cryostat
were used in the region of 700-180 cm-1. For each spectrum,
200 scans at a resolution of 0.25 cm-1 were coadded. The
gaseous sample was mixed with argon (1:1000) in a 1-L
stainless-steel storage container, and then small amounts (ca. 1
mmol) of the mixture were deposited within 30 min onto the
cold matrix support (16 K, Rh-plated Cu block) in high vacuum.
Details of the matrix apparatus have been described elsewhere.13

7626 J. Phys. Chem. A, Vol. 114, No. 28, 2010
Zeng et al.
of all atoms. The final refinement was obtained by introducing
anisotropic thermal parameters and the recommended weightings
for all of the atoms. The maximum electron densities in the
final difference Fourier map were located near the heavy atoms.
All calculations were performed using the SHELXTL-plus
package for the structure determination and solution refinement
and for the molecular graphics.23
Results and Discussion
Quantum Chemical Calculations. In order to locate the
minimum on the potential energy surface (PES), energy scans
were performed at the B3LYP/6-311+G(3df) level of theory
by changing the dihedral angles φ(FS-NN) in FSO2N3 and
φ(CS-NN) in CF3SO2N3 about the S-N bonds (Figure 1).
Surprisingly, only one minimum with the azide group in nearly
eclipsed position to one of the SdO bonds was found for both
molecules, which corresponds to conformer b shown in Scheme
3, while the two Cs symmetric conformers a and c are transition
states on the PES. The energy difference between conformers
b and a is significantly larger for CF3SO2N3 (26 kJ mol-1) than
that for FSO2N3 (14 kJ mol-1), likely as a consequence of the
steric repulsion between the CF3 and N3 groups. The energy
differences between conformers b and c are very similar for
FSO2N3 (8 kJ mol-1) and CF3SO2N3 (10 kJ mol-1).
Figure 2. Optimized structure of FSO2N3 and CF3SO2N3.
TABLE 1: Calculated and Experimental Structural
a
Parameters of FSO2N3
X-ray
BP86 MPW1PW91 crystallography
parametersb
B3LYP
r(S-O1)
1.419
1.411
1.578
1.672
1.247
1.117
124.4
99.1
111.4
106.1
115.3
173.1
1.434
1.426
1.603
1.697
1.252
1.135
1.412
1.404
1.561
1.656
1.241
1.113
1.411(2)
1.404(2)
1.547(2)
1.648(3)
1.270(4)
1.108(4)
122.82(15)
99.68(14)
112.76(15)
106.64(15)
113.0(2)
173.3(3)
-14.8(3)
-152.4(2)
97.9(2)
r(S-O2)
r(S-F)
r(S-N1)
r(N1-N2)
r(N2-N3)
∠(O1SO2)
∠(FSN1)
∠(O1SN1)
∠(O2SN1)
∠(SN1N2)
∠(N1N2N3)
124.8
124.3
98.7
111.5
105.5
115.1
172.6
-29.9
-168.3
81.4
99.2
111.2
106.3
115.1
173.5
-27.8
-165.9
83.5
The molecular structures for FSO2N3 and CF3SO2N3 with
conformation b (Figure 2) were fully optimized with different
DFT (B3LYP, BP86, and MPW1PW91) methods, and the
obtained structural parameters are listed in Table 1 and Table
2, respectively. Similar structural parameters were obtained for
all three DFT methods and the following discussion will only
refer to the B3LYP results. The most striking feature for both
molecules is the synperiplanar configuration of the N3 group to
one of the SdO bonds in FSO2N3 (φ(O1S-N1N2) ) -28.8°)
and CF3SO2N3 (φ(O1S-N1N2) ) -19.3°), and the synperipla-
nar SdO bonds for both azides are calculated to be longer than
the antiperiplanar ones by 0.008 Å at the B3LYP/6-311+G(3df)
level of theory. The CF3 group in CF3SO2N3 was calculated to
be staggered with respect to the SO2N moiety, which is in
agreement with steric arguments.
φ(O1S-N1N2) -28.8
φ(O2S-N1N2) -167.0
φ(FS-N1N2)
82.4
φ(SN1-N2N3)
176.1
176.0
175.8
168(3)
a The 6-311+G(3df) basis set was applied for all the calculations.
b Bond lengths and angles are given in angstroms and degrees,
respectively; for labeling of atoms see Figure 2.
an empirical absorption correction were completed by using the
CrysAlisPro program.22
(c) Solution and Refinement of the Structure. The solutions
were obtained by direct methods which located the positions
a
TABLE 2: Calculated and Experimental Structural Parameters of CF3SO2N3
X-ray
crystallography
parametersb
B3LYP
BP86
1.443
MPW1PW91
GEDc
r(S-O1)
1.428
1.420
1.876
1.692
1.331
1.324
1.323
1.246
1.117
123.9
110.6
106.0
99.8
1.421
1.413
1.856
1.673
1.323
1.316
1.315
1.240
1.113
123.9
110.5
106.1
99.82
115.0
173.9
-19.5
-156.4
92.5
1.418(2)
1.4153(13)
1.4085(13)
1.832(2)
1.6482(15)
1.319(2)
1.316(2)
1.313(2)
1.276(2)
1.106(2)
r(S-O2)
1.436
1.899
1.724
1.342
1.336
1.334
1.250
1.136
r(S-C)
1.849(6)
1.668(4)
1.321(3)
r(S-N1)
r(F1-C)
r(F2-C)
r(F3-C)
r(N1-N2)
r(N2-N3)
∠(O1SO2)
∠(O1SN1)
∠(O2SN1)
∠(CSN1)
1.252(7)
1.120(6)
124.3
123.02(9)
112.39(7)
105.80(8)
101.33(9)
111.99(12)
173.52(18)
-23.74(15)
-160.54(12)
88.49(14)
176.2(16)
110.7
105.3
99.4
114.8
173.3
-19.1
-156.0
93.1
111.1(22)
106.9(22)
107.5(11)
111.6(15)
173.1(35)
-24(6)
∠(SN1N2)
∠(N1N2N3)
φ(O1S-N1N2)
φ(O2S-N1N2)
φ(CS-N1N2)
φ(SN1-N2N3)
115.3
173.6
-19.3
-156.1
92.8
89(6)
172.2
172.6
172.4
a The 6-311+G(3df) basis set was applied for all the calculations. b Bond lengths and angles are given in angstroms and degrees,
respectively; for labeling of atoms see Figure 2. c Reference.4a

Anomeric Effects in Sulfonyl Compounds
J. Phys. Chem. A, Vol. 114, No. 28, 2010 7627
Figure 4. Matrix-IR spectra of FSO2N3 (absorbance A, resolution 0.25
cm-1) in the range of 1500-1100 cm-1. Lower trace: FSO2N3; Middle:
1:1 mixture of FSO215NNN and FSO2NN15N; Upper: 1:2.7:1.9 mixture
of FSO2N3, FS(16O)(18O)N3, and FS18O2N3.
Figure 3. Upper trace: IR spectrum of FSO2N3 isolated in an Ar-
matrix at 16 K (absorbance A, resolution 0.25 cm-1). Middle trace: IR
spectrum of gaseous FSO2N3 at 300 K (transmission T, resolution 2
cm-1). Lower trace: Raman spectrum of liquid FSO2N3 at 300 K
(Raman intensity I, resolution 2 cm-1). Bands associated with impurity
of CH3NO2 are marked with an asterisk.
cm-1. The correct assignment of the experimental bands at
1224.1 and 1177.4 cm-1 to the respective sym. stretches of N3
and SO2 was confirmed by their 16/18O and 14/15N isotopic shifts
(Figure 4).
Most of the bands in the matrix IR spectrum are flanked by
bands attributable to different matrix sites with frequency
differences within a few wavenumbers (<5 cm-1); no evidence
for the existence of a second conformer could be obtained. The
gas-phase IR and liquid-phase Raman spectra also show no
additional bands that correspond to a second conformer.
Therefore, in accordance with the computational results, only
one conformer is present in the gas and liquid states, as well as
in the Ar matrix.
Vibrational Spectroscopy. The Ar-matrix IR, gas-phase IR,
and liquid-state Raman spectra were recorded of FSO2N3 and
are shown in Figure 3. The experimental frequencies are listed
in Table 3 together with the vibrational frequencies obtained
from B3LYP/6-311+G(3df) calculations and their corresponding
assignments.
The N3 and SO2 asymmetric stretches appeared at 2162.3 and
1466.2 cm-1 in the Ar-matrix IR spectrum, respectively, which
are in good agreement with the corresponding calculations of
2287 and 1474 cm-1 by B3LYP/6-311+G(3df). In contrast to
the experimental observations, the order of the symmetric
stretches of these two groups was interchanged by all three DFT
methods (B3LYP, BP86, MPW1PW91), e.g., the B3LYP/6-
311+G(3df) calculations (Table 3) predicted a higher frequency
for sym. N3 stretch at 1256 cm-1 than sym. SO2 stretch at 1230
According to the calculations, the two SdO bonds are
different by 0.008 Å for all three DFT methods (Table 1). Thus,
for the mixed-labeled 18OS 16O moiety two different frequencies
are expected for several fundamental vibrations in the matrix
TABLE 3: Experimental and Calculated Vibrational Frequencies (cm-1) of FSO2N3
experimentala
calculatedc
mode/assignment
IR (gas-phase)
IR (Ar-matrix)b
Raman (liquid)
2161 vs
1473 s
1230 s
1181 s
826 s
777 m
640 s
520 w, br
2162.3 s
1466.2 vs
1224.1 vs
1177.4 s
819.1 vs
775.4 m
635.9 s
549.3 m
518.2 m
496.0 m
475.6 m
350.2 w
2168 s
1450 w
1216 s
1171 vw
824 vw
774 vs
636 s
520 m
500 m
478 s
2287 (393)
1474 (234)
1230 (174)
1256 (274)
790 (247)
752 (59)
638 (152)
581 (6)
V1, N3 asym. stretch
V2, SO2 asym. sretch
V3, SO2 sym. sretch
V4, N3 sym. sretch
V5, S-F stretch
V6, S-N stretch
V7, in-plane N3 deformation
V8, out-of-plane N3 deformation
V9, SO2 bending
V10, FSO bending
V11
508 (14)
484 (10)
472 (6)
485 w, sh
364 m
353 (3)
V12
a Experimental band positions and intensities: vs, very strong; s, strong; m, medium strong; w, weak; vw, very weak; sh, shoulder; br, broad.
b Most intensive matrix site. c Calculated IR data at the B3LYP/6-311+G(3df) level with intensities (km mol-1) in parentheses. The remaining
last three fundamental vibrations are at 340 (1), 157 (1), 60 (<1) cm-1
.

7628 J. Phys. Chem. A, Vol. 114, No. 28, 2010
Zeng et al.
TABLE 4: Experimentala and Calculatedb 16/18O Isotopic
Shifts (cm-1) for FSO2N3
∆V(16/18O1)c
∆V(16/18O2)d
∆V(16/18O116/18O2)e
exptl calcd
exptl
calcd
exptl
calcd
V1
0.2
16.4
29.2
0.0
0.0
3.7
3.9
0.0
7.1
5.9
4.7
2.9
0.0
16.3
30.9
0.0
0.3
3.2
3.7
0.0
6.9
5.9
4.9
2.8
0.2
23.1
23.6
0.0
1.1
1.5
5.3
0.0
7.1
7.9
3.0
2.2
0.0
23.4
25.0
0.1
0.9
1.2
4.8
0.0
7.1
7.4
3.3
2.2
0.2
43.1
50.5
0.0
1.6
5.2
9.6
0.0
15.3
13.2
7.8
0.0
44.1
51.5
0.1
1.3
4.5
8.5
0.0
15.1
12.5
8.4
V2
V3
V4
V5
V6
V7
V8
V9
V10
V11
V12
5.2
5.1
a Observations in Ar matrix. b Calculations at the B3LYP/6-311+
G(3df) level of theory. c 18O isotopic labeling of O1 (Figure 2).
d 18O isotopic labeling of O2 (Figure 2). e 18O isotopic labeling of
both O1 and O2 (Figure 2).
Figure 5. IR spectrum of Ar-matrix isolated CF3SO2N3 at 16 K
(absorbance A, resolution 0.25 cm-1).
TABLE 5: Experimental and Calculated IR Frequencies
(cm-1) of CF3SO2N3
IR spectrum of FSO2N3. The IR spectra of Ar-matrix isolated
FSO2N3:FS(16O)(18O)N3:FS18O2N3 ) 1:2.7:1.9 in the range of
1500-1100 cm-1 are shown in Figure 4 (upper trace). For
comparison, the IR spectrum of 15N-labeled FSO2N3 (FSO215NNN:
FSO2NN15N ) 1:1) is also shown (Figure 4, middle trace), and
the 14/15N isotopic shifts are give in Table S1 (Supporting
Information). The experimental 16/18O isotopic shifts are listed
in Table 4, which show very good agreement with the calculated
values. Significant differences between the experimental and
predicted 14/15N isotopic shifts were observed for V1 (experi-
mental, 5.5 cm-1; calculated, 1.2 cm-1) and V11 (experimental,
1.6 cm-1; calculated, 3.1 cm-1), suggesting strong anharmonic
perturbations.
experimentala
calculatedc
mode/assignment
IR (gas phase) IR (Ar matrix)b
2152 s
1440 s
1237 s
1220 s, sh
2151.6 s
1434.6 s
1242.5 s
1231.7 s
1208.7 m
1162.3 m
1130.7 vs
786.6 m
756.6 m
628.5 m
2280 (444) V1, N3 asym. stretch
1438 (216) V2, SO2 asym. stretch
1255 (188) V3, CF3 asym. stretch
1229 (366) V4, SO2 sym. stretch
1213 (193) V5, CF3 asym. stretch
1191 (51) V6, N3 sym. stretch
1114 (261) V7, CF3 sym. stretch
770 (28) V8, CF3 deformation
737 (64) V9, S-N stretch
1168 s
1133 s
784 m
755 m
632 s
630 (197) V10, N3 in-plane deformation
579 (1)
V11, N3 out-of-plane deformation
585 m
584.0 m
577 (80) V12, SO2 deformation
551 (<1) V13, CF3 deformation
536 (14) V14, CF3 deformation
As is shown in Figure 4 (upper trace), the mono-18O-labeling
of the two oxygen atoms, O1 and O2 in Figure 2, lead to
different isotopic shifts for the vibrational modes of V2 (asym.
SO2 stretch) and V3 (sym. SO2 stretch). For the asymmetric
stretch, 18O-labeling of O1 causes a shift of 16.4 cm-1, which
is significantly smaller than the effect of 18O-labeling of O2
(23.1 cm-1). The corresponding isotopic bands for V2 show
different line shapes, probably due to vibrational coupling. Well-
separated bands were observed for the sym. SO2 stretch with
experimental 16/18O isotopic shifts of 29.2 and 23.6 cm-1. A
similar isotopic splitting pattern was also observed for the FSO
bending mode (V10) in the region of 530-460 cm-1 (Figure S1
in Supporting Information), but for the SO2 bending mode (V9),
only three bands appeared, indicating very close 16/18O isotopic
shifts (7.1 cm-1) for FS(16O1)(18O2)N3 and FS(18O1)(16O2)N3,
546 w
452 w
421 w
544.7 m
453.4 m
422.6 w
443 (2)
418 (4)
318 (0)
315 (0)
279 (1)
195 (2)
179 (3)
V15
V16
V17
V18
V19
V20
V21
207.1 vw
203.9 vw
190.0 vw
a Experimental band positions and intensities: vs, very strong; s,
strong; m, medium strong; w, weak; vw, very weak; sh, shoulder;
br, broad. b Most intensive matrix site. c Calculated IR data at the
B3LYP/6-311+G(3df) level with intensities (km mol-1) in paren-
theses.
(V9) for CF3SO2N3 appeared at 756.6 cm-1 and it is lower than
that of FSO2N3 at 775.4 cm-1 in Ar matrix, indicating a slightly
stronger and shorter S-N bond in the latter molecule.
which
is
consistent
with
the
calculated
iso-
topic shifts of 7.1 and 6.9 cm-1 for these two isotopomers,
respectively.
X-ray Crystallography. The two title azides, FSO2N3 and
CF3SO2N3, are liquid at room temperature; crystals suitable for
single-crystal X-ray crystallography were grown by slow
evaporation at -20 °C and condensation at -80 °C in an
evacuated L-shaped flame-sealed glass tube. The crystal-
lographic data for both compounds are listed in Table 6.
Fluorosulfonyl azide, FSO2N3, crystallizes in the monoclinic Cc
space group, with four molecules in the unit cell. The molecular
structure of the FSO2N3 molecule is shown in Figure 6, and the
structural parameters are given in Table 1. The C1 symmetric
molecule contains two nonequivalent oxygen atoms, the S1dO1
bond shows a nearly eclipsed conformation (synperiplanar) to the
N1-N2 bond with a torsion angle of -14.8(3)° (φ(O1S1-N1N2)),
while the S1dO2 bond exhibits an antiperiplanar conformation to
the N1-N2 bond (φ(O2S1-N1N2) ) -152.4(2)°). In agreement
The IR spectrum of Ar-matrix isolated CF3SO2N3 is shown in
Figure 5, the observed IR frequencies are listed in Table 5 and
compared with the gas-phase and calculated IR spectra. In
agreement with the previous IR (gas-phase) and Raman (liquid)
studies,4b the assignments of all observed vibrational bands (gas-
phase, liquid, and Ar-matrix) can be assigned in terms of a single
conformer.
The assignments of the Ar-matrix IR spectrum were aided
by 15N labeling experiments (Table S2 in Supporting Informa-
tion), but the vibrations are perturbed by anharmonic resonances
as suggested by the large differences between the observed and
calculated isotopic shifts for the vibrations involving the N3 and
SO2 moieties in CF3SO215NNN, i.e., V1, V4, V6. The S-N stretch

Anomeric Effects in Sulfonyl Compounds
J. Phys. Chem. A, Vol. 114, No. 28, 2010 7629
TABLE 6: Crystallographic Data of FSO2N3 and CF3SO2N3
SCHEME 4: Description of Possible Anomeric
Interactions in Sulfonyl Azides RSO2N3
chemical formula
space group
a (Å)
b (Å)
c (Å)
FN3O2S
Cc
7.0017(13)
13.5124(15)
5.0170(9)
90
121.37(3)
90
405.25(12)
4
CF3N3O2S
P21/c
5.0666(4)
17.0136(10)
7.0041(5)
90
106.130(7)
90
579.99(7)
4
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z (molecules/unit cell)
mol wt
125.09
2.050
175.10
2.005
calcd density (g cm-3
)
T (°C)
-123
-123
tively, and the former SdO bond (1.4153(13) Å) is longer than
the latter (1.4085(13) Å). The S1-N1 bond length in CF3SO2N3
is 1.6482(15) Å, which is very close to that in FSO2N3 (1.648(3)
Å). The angle (∠(SN1N2)) at the bridging nitrogen atom (N1)
is 111.99(12)°, and it is slightly smaller than that in FSO2N3
(113.0(2)°). The CF3 group is staggered to the SO2N moiety
along the C-S bond. The crystal packing of CF3SO2N3 is shown
in Figure S3 (Supporting Information); clearly the molecules
are aligned to have minimum dipole moment for the solid.
Conformational Properties. Recently, Besenyei et al.25 re-
ported the crystal structure of six arenesulfonyl azides and found
that the N3 group exhibits a synperiplanar conformation to one of
the SdO double bonds, while the other one is in an antiperiplanar
position. The SdO bond lengths are strongly perturbed by through-
space nonbonded interactions, thus no conclusive information could
be obtained for the subtle intramolecular electronic interactions.
In order to rule out any intermolecular effects on the conformation,
the IR spectrum of matrix isolated FSO2N3 combined with 18O-
labeling was recorded, which clearly showed the structural non-
equivalence of the two oxygen atoms in the molecule, ruling out
through-space nonbonded interactions as the cause for this con-
formation. In addition, the agreement between the gas-phase and
the solid-state structures of CF3SO2N3 indicates the negligible
effects of packing on the conformation. As a consequence, it can
be assumed that electronic (anomeric) effects are dominant in
determining the conformation and that the crystal structure of
FSO2N3 is also a good model for the gas-phase structure. These
assumptions are supported by the computational results (vide supra).
The theoretically predicted and also experimentally deter-
mined structures of FSO2N3 and related CF3SO2N3 revealed a
synperiplanar configuration of the longer SdO bond to the N3
group, while the shorter SdO bond is in an antiperiplanar
configuration. All of these structural features can be viewed as
a function of the anomeric interaction of nσ(N) f σ*(S-O),
i.e., the donation of electron density of the σ-type lone-pair on
the R-nitrogen nσ(N) into the antibonding orbital of the
synperiplanar (with respect to the N3 group) SdO bond,
σ*(S-O). As an additional result, larger angles of ∠O1SN1
are observed for both molecules by X-ray crystallography
(FSO2N3 112.76(15)°, CF3SO2N3 112.39(7)°) than ∠O2SN1
(FSO2N3 106.64(15)°, CF3SO2N3 105.80(8)°).
µ (mm-1
)
0.698
0.567
a
R1
0.0260
0.0649
0.0316
0.0673
b
wR2
a R1 is defined as Σ|Fo| - |Fc|/Σ|Fo| for I > 2σ(I). b wR2 is
defined as [Σ[w(Fo - Fc2)2]/Σw(Fo )2]1/2 for I > 2σ(I).
2
2
Figure 6. View of the FSO2N3 molecule in the unit cell. Thermal
ellipsoids are drawn at the 50% probability level.
Figure 7. View of the CF3SO2N3 molecule; thermal ellipsoids are
drawn at the 50% probability level.
with the optimized gas-phase geometry, the synperiplanar SdO
bond (1.411(2) Å) is significantly longer than that of the anti-
periplanar one (1.404(2) Å). The S-F bond length is 1.547(2) Å
and longer than those of F2SO2 (1.514(2) Å)24 and FSO2Cl
(1.5377(12) Å)24 in the solid state, consistent with the relative
electronegativity of the ligands N3 < Cl < F. These solid-state
structural features are generally reproduced by the DFT calculations
for the gas phase, and no significant through-space nonbonded
interactions were observed in the solid.
Trifluoromethylsulfonyl azide, CF3SO2N3, crystallizes in the
monoclinic P21/c space group, with four molecules in the unit
cell. The molecular structure is shown in Figure 7, the structural
parameters are listed in Table 2 and compared with the GED
results. The structural parameters of CF3SO2N3 in the solid state
are in overall excellent agreement with those obtained by GED
in the gas phase,4a but with much higher accuracy. The S1dO1
and S1dO2 bonds are synperiplanar and antiperiplanar to the
azide group with dihedral angle of φ(O1S1-N1N2) )
-23.74(15)° and φ(O2S1-N1N2) ) -160.54(12)°, respec-
In principle as shown in Scheme 4, two competing anomeric
interactions nσ(N) f σ*(S-F) and nσ(N) f σ*(S-O) in
FSO2N3, and nσ(N) f σ*(S-C) and nσ(N) f σ*(S-O) in
CF3SO2N3 are expected to determine the conformation of these
molecules. According to the RHF/6-31G(d) calculations, the
energies for the acceptor orbitals of σ*(S-F) (0.181 au) and
σ*(S-C) (0.243 au) are much higher than the other acceptor
orbital σ*(S-O) in FSO2N3 (0.111 au) and CF3SO2N3 (0.116
au), respectively, which renders the donation of σ-type nitrogen
lone pair, nσ(N), to σ*S-O more favorable.
The overlap between preorthogonal natural bond orbitals (PN-
BOs) reflects the strength of the associated NBO Fock matrix

7630 J. Phys. Chem. A, Vol. 114, No. 28, 2010
Zeng et al.
CF3SO2N3. This material is available free of charge via the
Internet at http://pubs.acs.org.
References and Notes
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Figure 8. Contour plots of the vicinal nσ(N) f σ*(S-O) interactions
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Conclusions
Vibrational spectroscopic studies of FSO2N3 in gas phase,
Ar matrix, and liquid state suggest the existence of only one
conformer, and the two oxygen atoms were supposed to be
structurally nonequivalent based on the matrix IR spectrum of
mixed 18O-labeled FSO2N3. These spectroscopic results were
confirmed by its structure determined by X-ray crystallography,
which shows that the azide group is synperiplanar to one SdO
group (φ(O1S-N1N2) ) -14.8(3)°) and antiperiplanar to the
other one (φ(O2S-N1N2) ) -152.4(2)°) with small but
significant differences in SdO bond lengths. Similar confor-
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CF3SO2N3 in both gas phase (GED) and solid state (X-ray
crystallography). The exclusive presence of the same sterically
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Acknowledgment. X. Z. expresses his thanks to the Alex-
ander von Humboldt Foundation for a research grant; H. B. and
H. W. acknowledge support from the Deutsche Forschungsge-
meinschaft and the Fonds der Chemischen Industrie and M.G.
acknowledges the University of Lethbridge for granting a study
leave. We are grateful to Dr. Wenda for providing (CF3SO2)2O.
Supporting Information Available: Matrix-IR spectra
(Figure S1) of FSO2N3 in the range of 530-460 cm-1; packing
diagrams of FSO2N3 (Figure S2) and CF3SO2N3 (Figure S3) in
the solid state; experimental and calculated 14/15N isotopic shifts
of FSO2N3 (Table S1) and CF3SO2N3 (Table S2); NBO second-
order perturbative estimates of interaction energies and NBO
occupancies (Table S3); X-ray crystallographic files in CIF
format for the structure determinations of FSO2N3 and
JP103616Q