Carbonyl diazide, OC(N3)2: Synthesis, purification, and IR spectrum

Article abstract of DOI:10.1021/ic301270b

Carbonyl diazide (1), OC(N₃)₂, is prepared by reaction of triphosgene and tetra-n-butylammonium azide in a solution of diethyl ether or dimethyl ether. The advantage of this synthetic method, relative to other procedures, is that the use of triphosgene, OC(OCCl₃) ₂, mitigates the need to use highly poisonous reagents such as phosgene, OCCl₂, or chlorofluorocarbonyl, OC(Cl)F. The identity and purity of OC(N₃)₂ are established by gas-phase IR spectroscopy, which reveals the presence of both syn-syn and anti-syn conformers. Computed anharmonic vibrational frequencies and infrared intensities of carbonyl diazide (1) display excellent agreement with experiment, and reveal the presence of strong Fermi resonances.

Full text of DOI:10.1021/ic301270b

Article
pubs.acs.org/IC
Carbonyl Diazide, OC(N3)2: Synthesis, Purification, and IR Spectrum
Alex M. Nolan,† Brent K. Amberger,† Brian J. Esselman,† Venkatesan S. Thimmakondu,‡
John F. Stanton,*,‡ R. Claude Woods,*,† and Robert J. McMahon*,†
†Department of Chemistry, University of Wisconsin−Madison, 1101 University Avenue, Madison, Wisconsin 53706-1322, United
States
‡Institute for Theoretical Chemistry, Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas
78712, United States
S
* Supporting Information
ABSTRACT: Carbonyl diazide (1), OC(N3)2, is prepared by reaction of triphosgene
and tetra-n-butylammonium azide in a solution of diethyl ether or dimethyl ether. The
advantage of this synthetic method, relative to other procedures, is that the use of
triphosgene, OC(OCCl3)2, mitigates the need to use highly poisonous reagents such as
phosgene, OCCl2, or chlorofluorocarbonyl, OC(Cl)F. The identity and purity of
OC(N3)2 are established by gas-phase IR spectroscopy, which reveals the presence of both syn−syn and anti−syn conformers.
Computed anharmonic vibrational frequencies and infrared intensities of carbonyl diazide (1) display excellent agreement with
experiment, and reveal the presence of strong Fermi resonances.
INTRODUCTION
■
Carbonyl diazide (1), OC(N3)2, is a highly explosive
compound that has been the subject of recent studies
concerning its synthesis, structure, and reactivity.1 Among
various interesting attributes, the compound is notable because
it is a high energy structure composed of very stable diatomic
species (CO + 3N2), with a computed specific enthalpy of
decomposition similar to industrial explosives TNT, RDX, and
HMX.2 Although known since the 1920s,3,4 the explosive
nature of carbonyl diazide (1) discouraged isolation attempts,
making the recent synthesis, purification, and characterization
by Zeng et al. quite remarkable.1
Recent interest in carbonyl diazide (1), CON6, stems from
its ability to serve as a precursor to other high energy molecules
on the CON4 and CON2 potential energy surfaces (Figure 1).
Photolysis of OC(N3)2 (1) in an inert matrix results in loss of
N2 to generate CON4 isomers: acyl nitrene N3−C(O)N (3)
and its Curtius rearrangement product N3−NCO (4).5
Remarkably, flash vacuum pyrolysis of OC(N3)2 (1) at 400
°C yields diazirinone (2),6−8 the C2v conjugate of molecular
nitrogen and carbon monoxide. An intriguing species in terms
of structure and bonding,9−11 diazirinone (2) has been the
subject of a number of theoretical studies and has received
recent attention because of its relevance to the chemistry of CO
and N2 under harsh reaction conditions. Kaiser and co-workers
recently suggested that irradiation of N2/CO ices generates
diazirinone (2) under astronomically relevant conditions.12
N2CO isomers are also known to form under ionizing
conditions in the gas phase,13 which presents the possibility
that diazirinone (2) or other N2CO isomers may be formed in
interstellar or circumstellar space. Mechanistic details concern-
ing the conversion of carbonyl diazide (1) to diazirinone (2)
remain unclear at this time.
Figure 1. Experimentally observed decomposition pathways for
carbonyl diazide, OC(N3)2.5−8
After our earlier studies established that a previous report of
the experimental observation of diazirinone (2) was incorrect,14
we turned our attention to the development of alternative
methods for generating and detecting this species. We
considered carbonyl diazide as a viable, if not especially
attractive, precursor to diazirinone (2). Carbonyl diazide (1)
was first synthesized in the 1920s via the diazotization of
carbohydrazide (5)3,4 (Figure 2, eq 1) but was not studied in
pure form due to its aforementioned explosive nature. In 2007,
carbonyl diazide (1) was produced through the incidental
hydrolysis of tetraazidomethane (6)15 in undried solvent
(Figure 2, eq 2) and was identified by 13C and 15N NMR
Received: June 15, 2012
Published: August 28, 2012
© 2012 American Chemical Society
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OC(N3)2 in diethyl ether in a stainless-steel flask was frozen at −196
°C, and the vessel was evacuated. The sample was allowed to warm to
room temperature, and all volatile materials were cryopumped to a
second stainless-steel flask at −196 °C to ensure that absolutely no
nonvolatile or particulate matter contaminated the sample. The second
flask was then held at −78 °C, a temperature at which Et2O is volatile
but OC(N3)2 is not, and the ether was removed with a diffusion pump
in conjunction with a liquid nitrogen trap. To remove all traces of
ether from the flask, it was necessary to perform three or four freeze−
pump−thaw cycles, wherein the sample flask was isolated from the
vacuum, warmed to room temperature, and cooled back to −78 °C
before pumping was resumed. Carbonyl diazide synthesized and
purified in this way typically afforded a yield of ca. 7.1 Torr-liter, (0.38
mmol, 19%).
Carbonyl Diazide (1): Method B. A modified version of the
preparation allows the entire procedure to be carried out on the
stainless-steel manifold. Use of dimethyl ether (bp −24 °C), which is
more volatile than diethyl ether (bp 35 °C), improves the ease and
efficiency of solvent removal from carbonyl diazide (1), although it
also requires that the reaction be performed at low temperature or
under pressure. Triphosgene (0.153 g, 0.516 mmol) and sodium azide
(0.207 g, 3.184 mmol) were added to a 35-mL stainless-steel flask. The
flask was attached to the stainless-steel manifold and evacuated, and
gaseous dimethyl ether was condensed into the vessel at −196 °C. In
the 35-mL flask, 1400 Torr-liter of dimethyl ether was calculated to
afford ∼4 mL as liquid solvent. After stirring for 16 h, the solvent was
distilled from the flask at −78 °C. The flask was then allowed to warm
to room temperature, and carbonyl diazide (1) was cryopumped to a
clean stainless-steel flask at −196 °C. Yields as high as 32% were
observed; however, this procedure proved more fickle than the
synthesis in diethyl ether.
Figure 2. Synthetic routes to carbonyl diazide, OC(N3)2.
spectroscopy. In 2010, Zeng et al. reported a synthesis of
carbonyl diazide from treatment of chlorofluorocarbonyl
ClC(O)F (7) with sodium azide by nucleophilic substitution
(Figure 2, eq 3).1,8 The product was isolated using trap-to-trap
distillation and thoroughly characterized by IR spectroscopy
(gas phase and argon matrix), Raman spectroscopy, and X-ray
crystallography. Here we describe an alternative procedure for
the synthesis of OC(N3)2 (1) using triphosgene (8) and tetra-
n-butylammonium azide (Figure 2, eq 4), which avoids the use
of chlorofluorocarbonyl ClC(O)F (7), a substance that is both
highly poisonous and not readily available.
EXPERIMENTAL SECTION
■
Caution! Carbonyl diazide (1) is shock-sensitive and explosive. Proper
safety precautions, which include the use of leather gloves, face shields, and
blast shields, must be taken when handling this compound.1,16,17 Sodium
azide (NaN3) and its hydrolysis product, hydrazoic acid (HN3) are
toxic.17 Sodium azide, as well as metal azides that may be formed by
reaction of sodium azide, represent explosion hazards.17
Characterization. Following either synthetic procedure, a small
portion of the sample was transferred to a stainless steel, gas-phase IR
cell (ca. 1 Torr) using vacuum line techniques. The purity of the
sample was assessed by comparison to spectral data described
previously by Zeng et al.1 A representative IR spectrum (Figure 3)
Many of the manipulations involved in the preparation and
purification of carbonyl diazide (1) were performed using a
stainless-steel vacuum manifold, which is described in greater detail
in Supporting Information. The purity of carbonyl diazide (1) samples
was examined by gas-phase IR spectroscopy, and percent yield was
estimated from the pressure in a stainless-steel manifold of known
volume. Unless noted otherwise, chemicals were acquired from
commercial sources and were used without further purification.
Tetra-n-butylammonium Azide. (TBAN3) was prepared using a
literature procedure and recrystallized from toluene.18 On occasion, a
residual toluene contaminant in TBAN3 was carried through the
subsequent synthetic procedures and could not be removed from the
desired carbonyl diazide. In order remove the toluene contaminant, a
slurry of TBAN3 (ca. 5 g) in 75 mL of diethyl ether was brought to
reflux with stirring. The slurry was then cooled to room temperature;
crystals were collected by filtration and washed with cold diethyl ether.
Residual diethyl ether solvent was removed from TBAN3 under high
vacuum.
Carbonyl Diazide (1): Method A. Tetra-n-butylammonium azide
(TBAN3) (1.64 g, 5.77 mmol) was added to an oven-dried round-
bottom flask, followed by anhydrous diethyl ether (10 mL) under
positive nitrogen pressure. The volume of solvent was not sufficient to
completely dissolve the ammonium salt. Triphosgene (194 mg, 0.655
mmol) was added, and the suspension was stirred for 2 h. The solution
was then passed through a silica plug to remove the suspended
ammonium salts, and diethyl ether (5 mL) was used to wash the silica.
The organic layer was washed with saturated aqueous sodium chloride
(10 mL) and dried over magnesium sulfate, and the drying agent
removed by gravity filtration.
Although a dilute solution of OC(N3)2 in diethyl ether seems
comparatively safe to handle in glass, a concentrated solution or the
isolated material is not. In one instance, a neat sample of OC(N3)2
detonated upon insertion of a glass pipet; the round-bottom flask
containing the sample was pulverized by the blast. Therefore, all
manipulations following the removal of the magnesium sulfate drying
agent were conducted in a 200-mL stainless-steel flask. The solution of
Figure 3. Infrared spectrum of gaseous carbonyl diazide, OC(N3)2 (1)
(1 Torr, 298 K, spectral resolution of 2 cm−1).
displays excellent agreement with literature data and is devoid of IR
absorptions attributable to impurities (see discussion below). Carbonyl
diazide can be stored at room temperature in a stainless steel flask for a
few weeks with minor decomposition. Samples of OC(N3)2 must be
handled gently, as physical shocks occasionally resulted in audible
explosive decompositions within the steel flasks.
RESULTS AND DISCUSSION
Synthesis. A logical strategy for the preparation of carbonyl
diazide (1) involves the use of phosgene or a “phosgene
■
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equivalent”. Given the serious hazards associated with the use
of phosgene, triphosgene has supplanted phosgene as the
electrophile of choice in reactions of this type.19,20
Triphosgene, OC(OCCl3)2, is a stable crystalline solid, which,
in solution, serves as a surrogate for 3 equiv of phosgene.
Although we have not engaged in a detailed mechanistic study
of this reaction, the transformations depicted in Scheme 1 have
an aqueous wash of the reaction mixture to ensure that any
remaining starting materials are safely neutralized and removed.
The final low-temperature, low-pressure distillation of solvent
from the carbonyl diazide (1) product is performed in a
stainless-steel vessel, which increases the quantity of pure
OC(N3)2 that may be handled safely. A disadvantage of this
procedure stems from the relative volatilities of carbonyl diazide
(1) and diethyl ether. Removing all of the solvent, without
losing an excessive amount of product, necessitates a series of
freeze−pump−thaw cycles. This procedure can be safely
performed with the stainless-steel apparatus, but it is tedious.
The alternate procedure involving triphosgene, sodium azide,
and dimethyl ether offers a different set of advantages and
disadvantages. The procedure is carried out entirely within a
stainless-steel apparatus, which enhances the margin of safety
although diminishes the ability to monitor the reaction in real
time. The higher volatility of dimethyl ether facilitates solvent
removal, but upon solvent removal, the neat product remains
mixed with solid sodium azide. This procedure afforded both
the greatest quantity and highest chemical yield of carbonyl
diazide (1). That the procedure is fickle may reflect an
instability of the mixture of neat carbonyl diazide and sodium
azide, an observation noted independently by Zeng et al.22
Although we attempted the synthesis using tetra-n-butylammo-
nium azide in dimethyl ether, we found the procedure to be
complicated by experimental difficulties associated with trans-
ferring 2 g of the highly hygroscopic TBAN3 to the 35-mL
stainless-steel reaction vessel and ensuring that the viscous mass
dissolved in dimethyl ether.
Without mitigating the dangers inherent in the preparation
and handling of carbonyl diazide (1), our procedure addresses
the complications associated with the literature procedure for
the synthesis of this compound:1 (i) the use of chlorofluor-
ocarbonyl (7), which is highly poisonous and is not
commercially available, (ii) the use of sodium azide under
heterogeneous (gas/solid) reaction conditions, (iii) a reaction
time of two hours vs four days, and (iv) the ability to
quantitatively assess product yield.
Potential Energy Surface. Optimized geometries, ener-
gies, and infrared intensities were obtained using the CCSD
method23 with the cc-pVDZ24 basis set as implemented in
Gaussian 09.25 The nature of stationary points was confirmed
by calculation of the harmonic vibrational frequencies, which
Scheme 1. Preparation of Carbonyl Diazide (1) Using
Triphosgene
good precedent in the literature.21 Sequential nucleophilic
attack of azide on the carbonyl carbon of triphosgene results in
the formation of 1 equiv of OC(N3)2 (1) and the liberation of 2
equiv of trichloromethoxide ion. Trichloromethoxide ion
rapidly eliminates chloride ion to generate phosgene. Phosgene
generated in situ is more reactive than triphosgene, and
subsequent reaction with azide yields additional OC(N3)2. In
our initial studies, we used sodium azide as the source of azide
ion and diethyl ether as solvent. Although these conditions
were successful in generating carbonyl diazide (1), we reasoned
that the very low solubility of sodium azide in diethyl ether was
a factor in the slow conversion of starting material and low yield
of product. We turned to tetra-n-butylammonium azide
(TBAN3) as a source of azide ion with greater solubility in
organic solvents. Use of TBAN3 enhanced the conversion of
triphosgene to carbonyl diazide (1) while minimizing the risk of
an explosive decomposition of solid sodium azide. Employing
standard solution-phase reaction conditions also allowed the
use of standard procedures to work up the reaction, including
Figure 4. Potential energy surface of OC(N3)2. Relative energies (kcal/mol, including ZPVE) at CCSD/cc-pVDZ (upper, black) and CCSD(T)/cc-
pVTZ (lower, red).
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Figure 5. Infrared spectrum of gaseous carbonyl diazide, OC(N3)2 (1 Torr, 298 K) with computed vibrational frequencies and intensities of syn−syn
(red) and anti−syn (blue) conformers superimposed as stick spectra. The y-axis scales for the computed infrared intensities have been adjusted to
reflect a qualitative agreement with the experimental spectrum. Several weak absorptions arise from the IR cell.
also provided zero-point vibrational energy (ZPVE) correc-
tions. The transition states were connected to their minima via
IRC calculations. CCSD(T)/cc-pVTZ26 optimized geometry
and harmonic frequency calculations were performed using
CFOUR27 for each of the minima. Further details of the
calculations can be found in Supporting Information.
The conformational isomers of carbonyl diazide (1) [syn−syn
(C2v), anti−syn (Cs), and anti−anti (C2)] are depicted in Figure
4. In good agreement with recent literature data,1,2 the lowest
energy conformer is syn−syn, with the anti−syn conformer lying
1.8 kcal/mol higher in energy at the CCSD(T)/cc-pVTZ level
of theory. The anti−anti conformer does not adopt an idealized
C2v structure; it exhibits a slight distortion from planarity (C2)
and lies ca. 11 kcal/mol higher in energy than the syn−syn
rotamer. The barriers to rotation of the N3 groups are modest
(Figure 4).
IR Spectrum. The gas-phase infrared spectrum of carbonyl
diazide (1) has been described previously,1 and we rely on the
IR spectrum as the criterion of purity for our synthetic method.
As such, it was of some concern to us that the computed
harmonic vibrational frequencies and infrared intensities for
syn−syn and anti−syn conformers of carbonyl diazide (1) do
not account for all of the prominent features in the
experimental spectrum, notably 2297, 2171, and 1200 cm−1.
The origin of these features is unclear. While it is possible that
these absorptions may arise from carbonyl diazide (1) through
the effects of anharmonicity (combinations, overtones, Fermi
resonance), it is also possible that they may be attributable to
unidentified impurities. We sought to explore this matter, in
greater detail, through a more sophisticated theoretical
treatment of the vibrational spectrum.
infrared intensities. For each of the three conformational
isomers of carbonyl diazide (1), geometries were optimized at
CCSD(T)/ANO0 and CCSD(T)/ANO1 levels of theory using
analytic gradients,28,29 and harmonic frequencies were obtained
with analytic second derivatives.30 The cubic and quartic force
constants were then calculated by numerical differentiation of
analytic second derivatives calculated at displaced points,
following the approach of Stanton et al.31 Second-order
vibrational perturbation theory (VPT2) was then used to
compute the fundamental vibrational frequencies as well as the
infrared intensities.32,33 Fermi resonances were analyzed using
the approach of Matthews and Stanton.34
In contrast to the harmonic analysis of the infrared spectrum,
which cannot account for many of its most prominent features,
the anharmonic analysis is in striking agreement with
observation (Figure 5, Tables 1 and 2). The cluster of features
seen from 2000 to 2300 cm−1 results from a strong and
extensive Fermi resonance in the syn−syn conformer of
OC(N3)2, whereby the combination levels ν4 + ν14, ν4 + ν15,
and ν3 + ν15 borrow intensity from the very strong asymmetric
NNN stretching mode, ν13. Similarly, the intrinsically
strong ν14 asymmetric CN stretching mode mixes with the
combination level ν7 + ν15. When these resonances are carefully
treated within the context of VPT2, a confident assignment
(rightmost column in Table 1) may be made for all of the
principal features seen in the spectrum (as well as most of the
minor features). It is rather remarkable that almost half of the
vibrational levels with predicted infrared intensities above 10
km mol−1 (5 of 12) correspond to two-quantum overtone and
combination levels, an observation that underscores the
problems that are associated with relying on the harmonic
treatment for the analysis of the spectrum.
To help analyze the puzzling aspects of the infrared
spectrum, we undertook high-level calculations that include
the effects of anharmonicity on the vibrational frequencies and
The experimental infrared spectrum also reveals the presence
of the anti−syn conformer of OC(N3)2 as a minor component
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Table 1. Experimental and Computed Vibrational
Table 2. Experimental and Computed Vibrational
Frequencies and Infrared Intensities for the syn−syn
Frequencies and Infrared Intensities for the anti−syn
a
a
Conformer of OC(N3)2
Conformer of OC(N3)2
fc-CCSD(T)
fc-CCSD(T)
harmonic
ANO0
harmonic
ANO1
VPT2+F
ANO1
harmonic
ANO0
harmonic
ANO1
VPT2+F
ANO1
mode sym
exptl
mode
sym
exptl
Fundamentals
2228 (63)
1764 (194)
1266 (5)
953 (20)
553 (0)
Fundamentals
2226 (308)
2206 (552)
1801 (482)
1290 (736)
1237 (374)
1125 (147)
901 (7)
1
a1
a1
a1
a1
a1
a1
a1
a2
a2
b1
b1
b1
b2
b2
b2
b2
b2
b2
2214 (54)
1782 (194)
1254 (4)
945 (19)
553 (0)
2191 (31)
1731 (179)
1222 (4)
933 (16)
544 (0)
1
2
3
4
5
6
7
8
9
a′
a′
a′
a′
a′
a′
a′
a′
a′
a′
a′
a′
a′
a″
a″
a″
a″
a″
2212 (295)
2192 (528)
1813 (475)
1279 (766)
1226 (354)
1114 (148)
894 (7)
2169 (174)
2142 (167)
1760 (198)
1243 (583)
1190 (177)
1083 (80)
884 (6)
2172
2145
1756
1257
1200
1082
2
1721
941
3
4
5
6
407 (2)
408 (2)
403 (1)
7
122 (0)
122 (0)
114 (0)
8
569 (0)
570 (0)
557 (0)
727 (34)
611 (2)
730 (33)
613 (2)
722 (30)
600 (2)
9
152 (0)
155 (0)
151 (0)
10
11
12
13
14
15
16
17
18
733 (29)
580 (9)
730 (25)
581 (7)
720 (28)
571 (8)
723
569
10
11
12
13
14
15
16
17
18
495 (1)
497 (1)
491 (1)
433 (1)
436 (1)
430 (1)
75 (1)
76 (1)
72 (1)
200 (2)
200 (2)
199 (2)
2198 (858)
1266 (1755)
1095 (17)
807 (23)
500 (2)
2212 (875)
1277 (1735)
1107 (13)
808 (26)
503 (3)
2140 (282)
1230 (1633)
1069 (4)
795 (21)
496 (1)
2145
1235
127 (0)
126 (0)
127 (0)
720 (27)
576 (2)
718 (24)
576 (1)
712 (18)
567 (2)
794
566 (11)
135 (0)
568 (10)
138 (0)
556 (7)
134 (0)
206 (1)
206 (1)
204 (1)
91 (0)
94 (0)
95 (0)
Two-Quantum Levels
Two-Quantum Levels
3 + 14
3 + 15
4 + 14
4 + 15
7 + 15
b2
b2
b2
b2
b2
2435 (11)
2286 (93)
2171 (441)
1998 (12)
1181 (40)
2450
2295
2172
2005
1200
2(5)
a′
a′
a′
a′
a′
a′
a′
a′
a′
a′
a′
a′
a′
2384 (23)
2321 (40)
2272 (28)
2192 (31)
2118 (54)
1797 (55)
1768 (178)
1269 (47)
1215 (24)
1199 (156)
1133 (73)
1127 (40)
1116 (13)
4 + 6
5 + 6
2(6)
4 + 7
6 + 8
2(7)
a
Computed harmonic and Fermi-resonance corrected (VPT2+F)
frequencies (cm−1), along with intensities given in parentheses (km
mol−1). Two-quantum (overtone and combination) levels with
intensity greater than 10 km mol−1 also included. Computed
frequencies are not scaled. Assigned experimental levels in italics.
1756
14 + 16
8 + 10
2(9)
1200
1138
1138
1138
2(15)
15 + 16
2(16)
of the gaseous mixture. The anharmonic analysis suggests that
the observed CO stretching frequency at 1756 cm−1 is
composed of contributions from both the fundamental, ν3, and
an overtone, 2ν7 vibration (Table 2). The most intense
vibration of the anti−syn conformer (ν4, 1257 cm−1) appears as
a shoulder on the high-energy side of the C−N stretching
vibration of the syn−syn conformer (ν14, 1235 cm−1).
Fundamental, combination, and overtone bands of the anti−
syn conformer contribute to absorptions observed at 1200,
1138, and 1082 cm−1. We also computed the anharmonic
infrared spectrum of the anti−anti conformer (see Supporting
Information section), but this conformer does not contribute to
the observed spectrum.
a
Computed harmonic and Fermi-resonance corrected (VPT2+F)
frequencies (cm−1), along with intensities given in parentheses (km
mol−1). Two-quantum (overtone and combination) levels with
intensity greater than 10 km mol−1 also included. Computed
frequencies are not scaled. Assigned experimental levels in italics.
intensities. After including the effects of anharmonicity, the
computed vibrational frequencies and infrared intensities of
carbonyl diazide (1) display excellent agreement with experi-
ment, and reveal the presence of strong Fermi resonances.
ASSOCIATED CONTENT
* Supporting Information
■
SUMMARY
S
■
Carbonyl diazide, OC(N3)2 (1), serves as a precursor to several
unusual, high-energy molecules; a safe and reliable preparation
for this substance represents an important objective. Our
synthesis provides advantages over previous methods in terms
of ready availability of starting materials, scalability of the
procedure, and, most importantly, safety. The infrared
spectrum of OC(N3)2 serves as the criterion of purity for
most synthetic procedures, which forced us to analyze this
spectrum in considerable detail. The spectrum is not well-
described by computed harmonic vibrational frequencies and
Description of stainless-steel vacuum manifold used for
synthesis and purification. Gas-phase IR spectra of OC(N3)2
obtained using various preparations. Details of computational
results at CCSD/cc-pVDZ and CCSD(T)/cc-pVTZ levels of
theory, including computed energies, zero-point vibrational
corrections, and Cartesian coordinates. Harmonic (CCSD(T)/
ANO0, CCSD(T)/ANO1) and Fermi-resonance corrected
(VPT2+F) frequencies and infrared intensities for the anti−
anti conformer of OC(N3)2. This material is available free of
charge via the Internet at http://pubs.acs.org.
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N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.;
Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;
AUTHOR INFORMATION
Corresponding Author
*E-mail: jfstanton@mail.utexas.edu (J.F.S.), rcwoods@wisc.edu
(R.C.W.), mcmahon@chem.wisc.edu (R.J.M.).
■
̈
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
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Notes
The authors declare no competing financial interest.
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1987, 87, 5968−5975.
(27) Stanton, J. F.; Gauss, J.; Harding, M. E.; Szalay, P. G. CFOUR,
Coupled-Cluster Techniques for Computational Chemistry; with con-
tributions from Auer, A. A.; Bartlett, R. J.; Benedikt, U.; Berger, C.;
Bernholdt, D. E.; Bomble, Y. J.; Cheng, L.; Christiansen, O.; Heckert,
M.; Heun, O.; Huber, C.; Jagau, T.-C.; Jonsson, D.; Juselius, J.; Klein,
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ACKNOWLEDGMENTS
■
We gratefully acknowledge the National Science Foundation
(CHE-1011959, R.J.M.) and the Robert A. Welch Foundation
(Grant F-1283, J.F.S.) for support of this project. We also thank
NSF for support of shared computing facilities at Wisconsin
(CHE-0840494).
Stopkowicz, S.; Tajti, A.; Vaz
integral packages MOLECULE (Almlof, J.; Taylor, P. R.), PROPS
(Taylor, P. R.), and ABACUS (Helgaker, T.; Jensen, H. J. Aa.;
Jørgensen, P.; Olsen, J.); and ECP routines by Mitin, A. V.; van
́
quez, J.; Wang, F.; Watts, J. D.; the
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