Molecular Structure of Isocyanic Acid, HNCO, the Imide of Carbon Dioxide

Article abstract of DOI:10.1021/acs.jpca.8b00557

Isocyanic acid, HNCO, the imide of carbon dioxide, was prepared by reaction of stearic acid and potassium cyanate (KOCN) at 60 °C in a sealed, thoroughly dried reactor. Interestingly, its crystal structure, solved by X-ray single crystal diffraction at 12

Full text of DOI:10.1021/acs.jpca.8b00557

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A: Molecular Structure, Quantum Chemistry, and General Theory
Molecular Structure of Isocyanic Acid HNCO, the Imide of Carbon Dioxide
Jürgen Evers, Burkhard Krumm, Quirin Josef Axthammer, Joern Martens, Peter Blaha,
Franz Xaver Steemann, Thomas Reith, Peter Mayer, and Thomas Matthias Klapoetke
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b00557 • Publication Date (Web): 07 Mar 2018
Downloaded from http://pubs.acs.org on March 8, 2018
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Molecular Structure of Isocyanic Acid HNCO,
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the Imide of Carbon Dioxide
Jürgen Evers,† Burkhard Krumm,† Quirin Axthammer,† Jörn Martens,† Peter Blaha,*††
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Franz Xaver Steemann,† Thomas Reith†, Peter Mayer,† Thomas M. Klapötke.*†
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† Department of Chemistry, Energetic Materials Research, Ludwig–Maximilian University of
Munich, Butenandtstr. 5–13 (D), D–81377 Munich, Germany.
†† Institute for Materials Chemistry, TU Vienna, Getreidemarkt 9/165–TC, A–1060 Vienna, Austria.
Email: tmk@cup.uniꢀmuenchen.de; peter.blaha@tuwien.ac.at
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required
according to the journal that you are submitting your paper to)
ABSTRACT: Isocyanic acid HNCO, the imide of carbon dioxide, was prepared by reaction of
o
stearic acid and potassium cyanate (KOCN) at 60 C in a sealed, thoroughly dried reactor.
Interestingly, its crystal structure, solved by X–ray single crystal diffraction at 123(2) K, shows a
group–subgroup relation for the NCOꢀ anion to carbon dioxide: (CO2: cP12, Pa–3, a = 5.624(2) Å,
150 K, C–O 1.151(2) Å; HNCO: oP16, Pca21, a = 5.6176(9), b = 5.6236(8), c = 5.6231(7) Å,
123(2) K). Precise positions of H, N, C and O were determined by DFT calculations with WIEN2k
leading to interatomic distances C–O 1.17, C–N 1.22, N–H 1.03, –N–H...N 2.14 Å and the
interatomic angle N–CꢀO 171°.
1. INTRODUCTION AND SYNTHESIS
In 1919 Irving Langmuir pointed out the extraordinary agreement between the physical properties
of carbon dioxide (CO2) and dinitrogen monoxide (N2O), including e.g. critical temperatures and
pressures, densities of the liquids, viscosities and refractive indexes and introduced the principle of
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isosterism.1 If compounds with the same number of atoms have also the same total number of
electrons, the electrons may arrange themselves in the same manner.1 In this case the compounds or
groups of atoms are said to be isosteric.1 For CO2 and N2O there are three atoms and 22 total or 16
valence electrons (e). Langmuir´s concept is extraordinary useful in predicting properties of
isosteric molecular species. About 26 isosteric species to CO2, either neutral or charged, are known
today.1ꢀ7
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Both, the three–atomic heterocumulenic cyanate and azide anions possess 22 total e, corresponding
to the weak acids isocyanic acid HNCO8 and hydrazoic acid HN3,9 respectively. In 1919 Langmuir
gave a first indication to their isosterism.1 Isocyanic acid was first prepared by Liebig and Wöhler10
in 1830. However, information on the molecular geometry on HNCO was obtained about 100 years
later in the decade 1940–1950 by electron diffraction,11 by microwave12 and infra–red
spectroscopy,13,14 all performed in the gas phase. As a result from those studies, the C–O, N–C and
N–H distances were determined to 1.17, 1.21 and 0.99 Å, respectively, with the hydrogen atom
being connected to nitrogen with an H–N–C angle of 128o. More recent studies regarding synthesis
and properties of HNCO were reported in our laboratory.15
In 1955 von Dohlen and Carpenter performed the first X–ray single crystal investigation. There,
o
HNCO crystallized at –125 C in an orthorhombic lattice, probably with space group Pnma.16 The
resulting distances C–O, N–C (1.18(2) Å, 2x) are in rough agreement with the earlier investigations
in the gas phase. However, the quality of the crystal was not sufficient enough to localize the
hydrogen atoms by difference Fourier analysis.
Apart from the interests of theoretical and synthetic chemistry for isocyanic acid, special
importance arises now due to its role in urban air pollution and possible toxicity. Cars with gasoline
and diesel engines are known to emit HNCO, especially if selective catalytic reduction systems are
used to reduce the emission of nitrogen oxides NOx. In this case an aqueous solution of urea
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((NH2)2CO) is added to the exhausts of the engine. In the catalytic reduction system urea is
thermally decomposed into ammonia (NH3) and isocyanic acid (HNCO). The last compound is
rapidly decomposed with water on the catalyst surface into ammonia and carbon dioxide. However,
there remains emission of HNCO in the range between 30 and 50 mg HNCO/kg–fuel17 which is the
found in the urban air pollution and could affect negatively human health.
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Of special interest is the comparison of the crystal structures of the isosteric acids HNCO and HN3.
The structure of HN3 was recently solved by X–ray single crystal investigation at 100(2) K18 and
shows a very interesting two–dimensional net, formed by bifurcated hydrogen bonds, in which
tetramers (HN3)4 were found in a nearly plane net of four–, eight– and sixteen–membered rings.
Now, a structural investigation of HNCO in the space group Pca21 is reported, including refinement
of the positional parameters by DFT calculations with the program WIEN2k.19
2. EXPERIMENTAL SECTION
2.1. General Information
Isocyanic acid HNCO was prepared from potassium cyanate KOCN and stearic acid as previously
reported.15 Both educts were intensively ground and filled in a Duran reactor which had at one side
a thinꢀwalled X–ray capillary. The reactor with the starting materials was dried at a high vacuum
line while cooling with liquid nitrogen traps.
2.2. X-Ray Crystallography
After several days of drying, the reactor was sealed from the vacuum line and then slowly heated to
o
60 C. Stearic acid melts near this temperature and then the formation of gaseous HNCO starts.
Isocyanic acid condenses at the capillary, which is cooled with liquid nitrogen and then sealed.
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From each heating experiment three capillaries filled with HNCO were obtained, which were stored
in liquid nitrogen.
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Single crystals of HNCO for the structure determination were grown in situ in the capillary adjusted
to the Oxford Xcalibur diffractometer. Several cooling and heating cycles are required to obtain a
single crystal in order to check the diffraction pattern. The structure was determined at 123(2) K.
The hkl ranges of the measured reflections were: h from –8 to 7, k from –8 to 8, l from –8 to 8.
Within a 24 h cooling and heating procedure from 100(5) to 185(5) K searching for the
orthorhombic phase of von Dohlen and Carpenter16 was negative. Structural calculations on HNCO
(Pca21) were performed with SHELXL–2014.20 The investigated crystal was disordered and
contained two individuals with 55% and 45%. The individuals were rotated against each other with
oxygen and nitrogen atoms nearly lying above each other. Therefore hydrogen atoms could not be
detected from a difference Fourier analysis. In addition, with the position derived by SHELXL–
2014 unrealistic short C–O and unrealistic long C–N distances had been obtained.
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2.3. Density Functional Theory (DFT) Calculations
The theoretical calculations are based on density functional theory (DFT) and were performed with
the augmented plane wave + local orbital method as implemented in the WIEN2k19 program. We
utilized the generalized gradient approximation of Perdew et al.21 and treated the weak but
important van der Waals interactions using DFTꢀD3.22 The atomic sphere radii RMT were chosen as
0.545 Å for C, 0.598 Å for N and O, and 0.333 Å for H. The plane wave cutoff parameter for the
wave functions, R*KMAX was set to 4 and the charge density and potential was expanded in plane
waves up to GMAX=20. For the calculations with Pca21 a 4x4x4 mesh was used, for that with Pnma a
2x4x8 mesh. We used experimental lattice parameters in all cases, but refined the positional
parameters of all atoms until the forces were well below 1 mRy/bohr. Since the position of the H
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atoms are unknown in many cases, we started from several educated guesses and refined to the next
local minimum. Clearly, the structure with H attached to N and bridging to another N atom has the
lowest total energy. The calculations were performed at the mpp2 cluster of the Leibniz–
Rechenzentrum, Garching, (Bavarian Academy of Science) and the Vienna Scientific Cluster
(VSC3).
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3. RESULTS AND DISCUSSION
3.1. X-Ray Diffraction Results
The results of a single crystal X–ray structure investigation are summarized in Table 1. HNCO
crystallizes in the orthorhombic acentric space group Pca21 with 4 formula units in the unit cell.
The positional parameters determined from the X–ray diffraction structure analysis by the program
SHELXL–201420 lead to both very short C–O ((0.99(1), 1.04(1) Å) and very long C–N ((1.30(1) and
1.36(1)) distances., since it is in space group Pca21 difficult to grow polarity pure single crystals.
Table 1: Crystallographic data of solid HNCO, determined at 123(2) K by X–ray single crystal
investigation. Structural analysis with the SHELXL–201420 program showed that the investigated
crystal consists of two crystals (1: 55 %, 2: 45 %), stacked above each other. Therefore, hydrogen
positions could not be precisely determined by a difference Fourier analysis.
Crystal system
Space group
a(Å)
orthorhombic
Pca21
5.6176(9)
5.6236(8)
5.6231(7)
123(2)
b(Å)
c(Å)
T (K)
V (Å3)
177.64(4)
4
Z
4 H, 4 N, 4 C, 4 O
ρcalc (g·cmꢀ3)
Reflections measured
4 a
1.609(4)
1390
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Reflections unique
339
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R(σ)
R1
0.0260
0.0352
0.0887
1.175
wR2
GOF
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Highest residual electron density
+0.10/−0.17
(e/Ǻꢀ3)
3.2. DFT Results
Therefore, it was decided to perform DFT calculations with the program WIEN2k.19 In Table 2 the
calculated positional parameters of solid isocyanic acid HNCO in the polar space group Pca21 are
summarized.
Table 2: Positional parameters x, y, z of solid isocyanic acid HNCO in space group Pca21 derived
by DFT calculations with the program WIEN2k.19 The C–, N– and O–atoms have been all located in
Wyckoff position 4a. Starting parameters where those obtained by the program SHELXL–2014.20
These atomic positions were then refined with WIEN2k.19 The positions of the H–atoms were
derived ab–initio. Details of the calculations are given in the Experimental Section.
Atom
C
x
y
z
0.0273
0.3329
0.8733
0.1755
0.2699
0.3770
0.1437
0.8520
0.0000
–0.6121
–0.9142
–0.2386
O
N
H
3.3. Group-Subgroup Relations
Solid carbon dioxide CO223 crystallizes at ambient pressure in the cubic space group Pa–3 with four
molecules in the unit cell. The lattice parameter is a = 5.624(2) Ǻ 23 at 150 K leading to a unit–cell
volume V = 177.88(20) Ǻ3. Solid isocyanic acid HNCO (Table 1) crystallizes in the orthorhombic
space group Pca21 with pseudo–cubic lattice parameters a = 5.6176(9), b = 5.6236(8), c = 5.6231(7)
Ǻ at 150 K. Hence, HNCO easily suggests a structural comparison of CO2 and HNCO in a group–
subgroup relation24ꢀ32 (Figure 1).33
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Coming from cubic CO2 (P21/a
–
3, No.205) the first step leads to a translationengleich24ꢀ31 (t) step
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of index 3 (t3) to the maximal non
position 24d in No. 205 is here reduced to 8c in No.61 while the axes have to be changed into c, b,
a and the coordinates into z, y, x. Further symmetry reduction is achieved in a translationengleich
step of index2 (t2) and an origin change with (0, 1/4, 0) to the maximal non isomorphic subgroup
Pca21. Here, position 8c in No.61 is changed into twice 4a in No.29 with changing the coordinates
into x, y+1/4, z. In Figure 1 the group subgroup relation is shown.
–isomorphic subgroup Pcab (P 21/c 21/a 21/b, No.61). The
–
–
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–
–
–
Figure 1. Group
symmetric cubic space group P21/a
to the acentric polar orthorhombic space group Pca21.(No 29).
–
subgroup relation24ꢀ31 showing the symmetry reduction for CO2 in the centro
3 (No.205) to Pcab (P21/c21/a 21/b, No.61) and then for HNCO
–
–
In CO2 the coordinates of the C atoms remain in position 4a (0,0,0), whereas the oxygen atoms O
are shifted from (0.1185, 0.1185, 0.1185) in Pa 3 to (0.1185, 0.1185, 0.1185) in P21/c 21/a 21/b.
–
–
For CO2 in Pca21 the C atoms have the coordinates (0, 1/4, 0), the O atom (0.1185, 0.3685,
and the Oi atom (
–0.1185)
–
0.1185, 0.1315, 0.1185).
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The symmetry reduction of Pa
–3 via Pcab to Pca21 (Figure 1) leads to two polar structures. The
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polar structure 1 is obtained with the z parameters (Table 2, z = –0.6121 and z = –0.9142) for the O
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and the N atom, respectively). In the polar structure 2 the z parameters are inverted to
z = 0.6121 and z = 0.9142 for O and N, respectively. The polar structure 2 is obtained from polar
structure 1 by reflection with a mirror.
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During the synthesis of HNCO with stearic acid and potassium cyanate (KOCN) and cooling to 100
K, the obtained sample will contain approximately the same number of crystals with polarity 1 and
with polarity 2. This could be a reason that the single crystal of HNCO used for the X
diffraction investigation was obtained only with low quality. In principle, the absolute configuration
of a single crystal in space group Pca21 with good quality, investigated by X ray diffraction, could
–ray
–
be determined by anomal dispersion.32 However, for elements with low scattering power as H, C, N
and O, the contribution is only low and therefore it is not an easy procedure to distinguish then
between such crystals.
In Figure 2 a Diamond view33 of the polar HNCO structure is shown with polarity 1 with negative
positional parameters z for the N, O and H atoms (Table 1). In this case the
Å in slightly zigzag chains have the direction “up” parallel to c. In the HNCO structure with
polarity 2 with positive positional parameters z for the O, N and H atoms the N H bonds have the
–N–H bonds with 1.026
–
direction “down”. In both polarities the z parameter of the C atoms is fixed to z = 0. Interestingly, in
H…N exist with 2.136 Å but none of N H…O,
HNCO short hydrogen distances of the type N
–
–
although oxygen is more electronegative than nitrogen. The N
–H
–
and the N
–
H…N bonds create
hydrogen bonds as one dimensional zigzag shaped chains along [001].
–
3.4. Crystal Chemical Properties
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Figure 2. Diamond view33 along [100] for the polar structure of HNCO in Pca21 is shown with
polarity 1 (negative positional parameters z for the O, N and H atoms (Table 1). The red N
have the direction “up”. In the structure with polarity 2 (positive positional parameters z for such
atoms), the red N H bonds have the direction “down”. Isotropic displacement spheres of 50 % are
shown with U=0.0300 Å2 for C, O, N and U = 0.0200 Å2 for H. The red N
H distances are 1.026 Å.
The fragmented red N
H…N hydrogen bonds are 2.14 Å. The chains of N H…N contacts are zigzag
–H bonds
–
–
–
–
shaped and run along [001]. The carbon atoms with z = 0 have the same coordinates.
Interatomic distances (Ǻ) and angles (o) for HNCO of previous investigations10,11,14 and those
obtained by this investigation with DFT calculations (WIEN2k19) are summarized in Table 3.
Table 3. Interatomic distances (Ǻ) and angles (o) determined by previous investigations in HNCO
by electron diffraction11, microwave and infra red12 in the gas phase and X ray diffraction16 in the
– –
solid state and those obtained by DFT calculations with WIEN2k.19
Experimental Technique
H–
N
N–
C
C
–O
H–
N
–
C
C
–
N
–O
–
N–
H…N
–N
–
H…N /
HꢀN…Hꢀ
electron diffraction11(gas phase)
1.01(est.)* 1.19
1.19
125 (est.)
microwave, infra
–
red12 (gas phase) 0.99(1)
1.21(1) 1.17(1) 128(1)
–
–
X
–
ray diffraction16(solid state)
1.18(2) 1.18(2)
this study, DFT, WIEN2k19
1.03
1.22
1.17
124
171
2.14
164 / 133
* estimated
The best agreement in Table 3 between the previous results and those obtained by DFT calculations
is achieved by Jones et. al.12 with microwave and infra
previous X
–
red investigations in the gas phase. In the
–
ray diffraction investigation16 the hydrogen atoms could not be detected due to the low
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crystal quality. Regarding the interatomic distances N
were obtained for both. However, in this study the C O distance (1.17 Å) is smaller than the N
red spectroscopic investigations in the
–C and C–O, identical values of 1.18(2) Å
–
–C
distance (1.22 Å), as also observed by microwave and infra
–
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gas phase 1.17(1) and 1.21(1) Å, respectively, by Jones et al.12
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In this investigation no evidence of an orthorhombic phase was found, which was observed by von
Dohlen and Carpenter.16 In addition to the structural investigation at 123(2) K (Table 1), a search
within 24 h for an orthorhombic phase was performed by measurements a 100(5), 120(5), 140(5),
160(5), 165(5), 170(5), 175(5), 180(5) and 185(5) K. In all cases only the slightly varying
orthorhombic lattice parameters in the range of 5.62 to 5.68 Å were obtained. Therefore, it must be
concluded that the phase of von Dohlen and Carpenter16 is metastable, if obtained by
depolymerization of cyanuric acid and not by protonation of cyanate by stearic acid. Also from
WIEN2k19 calculations it is evident that the total energy of the HNCO phase of von Dohlen and
Carpenter16 is about a few kJ/mole higher than that of the HNCO phase related to CO2.
Interestingly, a refinement of von Dohlen´s positional parameters16 by DFT calculations34 with
WIEN2k19 showed that in this phase N
–
H...O hydrogen bonds are present.
ꢀ
As already mentioned, the anions OCN/NCOꢀ and N3 are isosteric, but the crystal structures of
HNCO and HN318 differ significantly. In HNCO only one short
–N–
H...N hydrogen bond of 2.14 Å
is observed connecting the HNCO molecules to one dimensional zig zag chains along [001].
–
Hydrazoic acid contains 16 formula units in the unit
H...N hydrogen bonds are bifurcated into
H...Nshort 2.24(20) Å) and longer distances (Table 4, N H...Nlong
–cell, space group C1c1, and four
crystallographic independent HN3 molecules. Here, the N
–
shorter distances (Table 4, N
–
–
2.78(20) Å). With this connection a two
and sixteen membered rings are formed by (HN3)4 tetramers. Displacement ellipsoids show their
highest values perpendicular to the layers, indicating only weak van der Waals forces between
them. In Table 4 the average distances and angles in solid HN3 are summarized with labeling as H
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–dimensional layer structure is created, with four–, eight–
–
–
–
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NI–NII–
NIII.18
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5
6
7
Table 4. Average interatomic distances (Å) in HN3 for H
H...Nlong and average angles (o) H
NI NII, NI NII NIII, as determined by X
investigation.18
–
NI, NI
–
NII, NII
–
NIII, N
– ,
H...Nshort
N–
–
–
–
–
–ray single crystal
8
NII(o) NI
–
NII
–
NIII(o) N
–
H...Nshort(Å) N
–
H...Nlong(Å)
2.78(20)
9
H
–
NI(Å) NI NII(Ǻ) NII
–
–
NII(Å)
H
–
NI
–
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0.82(20) 1.233(4)
1.121(4)
109(4)
172.8(3)
2.24(20)
Upon comparison of the H
–
N interatomic distances in HNCO and HN3 according Tables 4 and 5, it
NII distance with
O distance with 1.17 Ǻ in HNCO is remarkably longer than the
is obvious that the N
–C distance with 1.22 Ǻ in HNCO is comparable to the NI–
1.233(4) Ǻ in HN3. However, the C
–
NII
–
NII distance with 1.121(4) Å in HN3. The angles N
–
C
–
O with 171° in HNCO and NI
–
NII
H distances. In
–ray diffraction, the
–NIII
with 172.8(3)o in HN3 deviate only by 2°. The strongest deviation is found for N
–
HNCO it was calculated by DFT to 1.03 Å, but in HN3, determined by X
averaged distance is 0.82(20) Å which is 0.21 Å smaller than in HNCO. The smaller value results
from the fact that the hydrogen atom in HN3 does not have a core electron.35 Therefore the
rays.35 A N
by WIEN2k19 in HNCO, can also be approximately assumed for HN3.
–N–H
distance is determined too short by X
–
–H distance of approximately 1.03 Å, as calculated
Although HNCO and HN3 are isosteric acids, there are small, but significant differences for the
melting and boiling points, as well as the acid constants pKa, as displayed in Table 5.
Table 5. Melting/boiling points and acid constants pKa for HNCO and HN3.
melting point (oC) boiling point (oC) acid constant pKa
HNCO
HN3
–
86.8,8
80,9
–
8636
8036
23.58, 2336
3.98, 3.736
4.89, 4.636
~
–
–
35.79, 35.736
The bonding in HNCO is to a small degree slightly weaker than in HN3. Therefore, the melting
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o
point for the first is approximately 6 C lower (Table 6). This tendency holds also for the boiling
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o
point of HNCO which is approximately 12 C lower than in HN3. The one
–dimensional chains in
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HNCO (Figure 3) seem to be slightly weaker than the two
structure formed by bifurcated hydrogen bonds.
–dimensional net in HN3 with a layer
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These structural facts are also reflected in the acid–base properties of HNCO and HN3. The pK acid
constant for HNCO is nearly one order of magnitude lower than in HN3, indicating a weaker
H...N bond and therefore weakly stronger acid properties for HNCO.
–N–
Although HNCO is slightly weaker hydrogen–bonded than HN3, the cell–volume for one formula
unit of HNCO at 1 bar (44.35 Å3, 123(2) K) is approximately 11 % lower than that for HN3 (49.68
Å3, 100(2) K). In this case the sizes of molecules with different composition are compared to each
other. However, one N–atom is also present in HNCO and the sizes of the remaining two atoms C
and O with covalent radii r for the rC = 0.77 Å and rO = 0.66 Å (with a sum of 0.77 + 0.66 = 1.43
Å)37can be approximately substituted by two N
= 1.40 Å).37 Since the layers in HN3 containing four
–atoms with rN = 0.70 Å (with a sum of 0.70 + 0.70
–
, eight– and sixteen–membered rings create
large voids, it can be expected that squeezing of HN3 (with HN3 structure) could probably lead to a
transformation into a more densely packed polymorph with HNCO structure.
However, as our own experimental experience shows, filling a high pressure diamond cell with
liquid HN3 at ambient pressure in a cooling room at T = –20°C is not an easy experimental task.
HN3 is a strong poison, very sensitive against moisture and has a high vapor pressure. The
difference of the boiling temperatures of diethylether (Et2O) and hydrazoic acid (HN3) is about
1°C.38 However, probably obtaining isotypic structures for HNCO at 1 bar and for HN3 at
–86 °C,
squeezed in a diamond cell, electrons may arrange themselves in the same manner, thus clearly
fulfilling Langmuir´s criterion for isosterism1 also for these solids.
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4. CONCLUSION
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5
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9
Isocyanic acid (space group Pca21), the imide of carbon dioxide (Pa/21
relations to CO2. The lattice parameters of orthorhombic HNCO (123(2) K) are pseudo
(Pca21) and deviate only on the second digit from that of cubic CO2. In solid CO2 isolated
molecules are bound to each other by only weak van der Waals bonds in a molecular structure.
–
3), shows strong structural
–
cubic
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–
–
From a topological projection of the crystal structure of CO2 on the crystal structure of HNCO
without hydrogen atoms it is shown that the C, N and O atoms are only slightly shifted. The
strongest shift is found for the carbon atoms with 0.15 Å, for the oxygen atoms O with 0.27 and for
the Oi and N atoms with
–0.18 Å, respectively. In HNCO short N
–
H...N hydrogen bonds (2.14 Å)
connect the molecules to one
–
dimensional zigzag chains. Due to the polar space group Pca21, there
are two structures for HNCO: e.g one with the N–H bond showing upwards in the chain, and the
other one downwards.
The comparison of the two isosteric acids shows that HNCO with chains of one
–dimensional
hydrogen bonds is slightly weaker bonded than HN3 with bifurcated hydrogen bonds in plane
layers, resulting in lower melting and boiling points for the first and also slightly stronger acid
properties.
–base
4. ACKNOWLEDGEMENTS
Financial support of this work by the LudwigꢀMaximilian University of Munich (LMU) is
gratefully acknowledged. The authors deeply thank Prof. H. Bärnighausen (Karlsruhe Institute of
Technology (KIT), Karlsruhe, Germany) for critical discussions and substantial improvements, as
well as Prof. Dr. W. Beck and Dr. F. Tambornino (Inorganic Chemistry, LMU) for critical support.
Computational support from Dr. M. Allalen (Leibniz
–
Rechenzentrum (LRZ), Garching, Germany)
is gratefully announced.
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Supporting Information. The cifꢀfile from the Xꢀray diffraction investigation for HNCO
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5
6
7
8
9
(Pca21), cifꢀfiles from the DFT calculations for HNCO (Pca21) and for HNCO (Pnma), and the
topological projection of the crystal structure of CO2 on the crystal structure of HNCO, are available
free of charge via the Internet at http://pubs.acs.org.
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34. Blaha, P., DFT calculation with WIEN2k for HNCO, unpublished results: Pnma, a = 10.82, b = 5.23, c = 3.57 Å, N
(0.4301, 1/4, 0.4041), C (0.3259, 1/4, 0.281), O (0.2192, 1/4, 0.2103), H (0.5128, 1/4, 0.2700).
35. Christe, K.; Wilson, W. W.; Dixon, D. A.; Khan, S. I.; Bau, R.; Metzenhin, T.; Lu, R. The Aminodiazonium Cation
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Et2O is 34.6 °C compared to 35.7 °C for HN3 (Table 5).
6.Table of Contents
Isocyanic acid HNCO is the imide of carbon dioxide, prepared by reaction of stearic acid and
potassium cyanate (KOCN) at 60 oC in a sealed, thoroughly dried reactor. Interestingly, the crystal
structure, solved by X
–ray single crystal diffraction at 123(2) K, shows a group–subgroup relation
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for the NCOꢀ anion to carbon dioxide.
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7. Figure Table of Contents
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