9
66
R.K. Khanna, M.H. Moore / Spectrochimica Acta Part A 55 (1999) 961–967
−
1
12
we assign the peak at 1292 cm
( C-residue)
H-bonding explained the stability and the temper-
ature dependence of vapor pressure of solid
−
1
13
which is shifted to 1277 cm on C-substitution,
−
to the COO symmetric stretch. Peaks below
200 cm are very similar to those in the spectra
H
2
CO
3
[5]. It is, therefore, proposed that hydro-
−
1
1
gen bonding in the solid phase stabilizes the zwit-
+
−
of NH HCO , (NH ) CO , NH NH CO and
ter-ion structure NH COO of carbamic acid.
3
4
3
4 2
3
4
2
2
+
3
−
+
2 3 3 2
NH CH COO , and are assigned to stretching
Both H CO and NH CO sublime by ꢀ250 K
2
−
at about the same rate, thereby indicating similar
hydrogen bond strengths.
and bending modes of the NCOO group (Table
).
Based on the above arguments we propose that
the stable structure associated with the residue is
2
As mentioned in the text, there is a very strong
resemblance between the IR spectra (4000–450
−
1
+
3
−
cm ) of residues (recorded at 250 K) and those
of NH HCO and (NH ) CO (recorded at 298
the zwitter-ion (NH COO ).
4
3
4 2
3
K). In as much as the identification of carbamic
acid (reported here for the first time) with the
residue appears reasonably convincing, in order to
firm up this identification, investigations of the
temperature dependence of the IR spectra of these
compounds are desirable. Also, far IR studies,
where lattice vibrations of carbamic acid,
NH HCO3 and (NH ) CO3 are expected to be
Quantum calculations for the zwitter-ion predict a
very long CN bond [2] which would result in
spontaneous dissociation into NH and CO even
3
2
at low temperatures. For H CO , quantum calcu-
2
3
lations predict its stability over that of H O+
2
−
1
4
4 2
CO by a 1–2 Kcal mol
[5,12]. Inclusion of
2
different, may provide check on this identifica-
tion. Such investigations are underway in our
laboratory.
Table 2
−
1
IR Absorption peaks (cm ) in the spectra of residues as-
signed as NH3 COO and NH3 COO
+
12
−
+13
− a
+
NH3 12COO− NH +3 13COO− Assignment
Acknowledgements
3
3
3
2
2
2
2
1
1
1
1
441
170
050
829
627
340
153
920
3170
2276
This work was supported by grants NAGW
3598 and RTOP 344-33-01 from NASA’s Plane-
tary Atmospheres Program.
NH–O stretch
Trapped CO2
−
References
595
441
330 (sh)
1567
1448
1324
COO asymmetric stretch
+
NH3 symmetric bend
[1] M. Renko, K.R. Liedl, B.M. Rode, J. Chem. Soc. Fara-
day Trans. 89 (14) (1993) 2375.
[2] W. Nanpeng, M.H. Brooker, J. Phys. Chem. 99 (1) (1995)
359–368.
[3] M.H. Moore, RK Khanna, Spectrochim. Acta. 47A (2)
(1991) 255.
[4] M.H. Moore, R.K. Khanna, B. Donn, J. Geophy. Res. 96
(1991) 17541.
+
(unresolved, vb)
1
NH3 asymmetric bend
319
−
1
1
9
8
8
8
7
6
5
292 (vb)
019
57 (sh)
83
1277 (sh)
1007(?)
970 (vb)
867(?, w)
COO symmetric stretch
CN stretch
+
NH3 coupled
Rock and Wag
66
33
64
83
−
808
780 (sh)
680
NCOO out of plane bend
[5] R.K. Khnnna, J. Tossell, K. Fox, Icarus 112 (1994) 541.
[6] N. DelloRusso, R.K. Khanna, M.H. Moore, J. Geophys.
Res. 98 (E3) (1993) 5505.
−
NCO bend
−
COO in plane bend
−
80
570 (?)
COO twist
[7] J.R. Bucarto, M.E. Palumbo, G. Strazzulla, Icarus 125
(
1997) 135.
a
Abbreviations: sh, shoulder; vb, very broad; w, weak, ?,
[8] W. Hage, A. Hallburcker, E. Myers, J. Chem. Soc. Fara-
day Trans. 9 (17) (1995) 2823.
too weak to accurately determine its frequency.