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“Isolated” N-H stretching frequencies in compounds containing NH2 groups
Structure, 82 (1982) 147-149 Elsevier Scientific Publishing Company, Amsterdam -
Journal of Molecular
Printed in The Netherlands
Short communication
“ISOLATED” CONTAINING
D. F. EVANS, Department
N-H STRETCHING NH2 GROUPS
FREQUENCIES
IN COMPOUNDS
P. H. MISSEN and M. W. UPTON
of Inorganic Chemistry,
Imperial College,
London
S?V7 2AY
(Gt. Britain)
(Received 8 December 1981)
Hydrogen bonding by N-H bonds is of some importance, not least in systems of biological interest. However, in many cases, the infrared spectra in the N-H stretching region exhibit very broad bands, even at low temperature (Fig. la)_ This limitation can often be overcome by extensive deuteration, so that isotopically dilute N-H bonds are present. In the absence of Fermi resonance, the number of isolated N-H stretches observabie should be equal to the number of chemically distinct N-H bonds. The isotopic dilution technique has been applied by Novak and his collaborators [ 1,2] (mainly at room temperature) to a number of compounds containing N-H bonds. Other applications.have been to solid hydrates [3] and ammonium salts [4] , while McKean
and his collaborators
[ 51 have made
extensive
studies
of compounds of the type CHD,X in the vapour phase. An excellent correlation between v(C-H) and r,-,(C-H) was observed. We have studied most molecules containing NH, groups, in the absence of NH or NH: groups, for which reasonably accurate neutron diffraction data are available [6 3 (standard deviation in r(N--H) < 0.006 a). The isolated N-H stretches observed at -90 K were found to correlate reasonably well with the chemically distinct N-H bonds present, and their internuclear distances_ The N-H stretches were distinguished from overtone or combination bands of the predominant fully deuterated species, u(N-H) of residual NH2 groups and C-H stretches, by varying the extent of deuteration in the approximate region W--97%. In urea, there are two pairs of equivalent
N-H
bonds.
HI \
N-Hz
0=C’ \ / HI
N-H2
These are involved in hydrogen bonds of very similar strength, and the N-H distances are indistinguishable within experimental error (0.998 + 0.005 A and 1.003 * 0.004 A) [7]. The infrared spectrum of -97% deuterated urea 0022-2860/82/0000-0000/$02.7
5 0 1982 Elsevier Scientific Publishing Company
148
80-
v (cm-‘)
60
201
3600
3400
3200
3000
u (cm-‘) Fig. 1. The infrared spectra at ca. 90 K of (a) urea, (b) deuterated (c) deuterated urea nitrate, protium ca. 7%.
urea, protium
ca. 3%
at ca. 90 K (Fig. lb) shows two strong sharp bands at 3367 cm-’ and 3394 bonds. cm-‘, which can be assigned to the two different types of N-H At room temperature, only a single, rather broad band was observed, as previously found by Lautie et al. [2]. In urea nitrate, (NH,),COWNO;, all four N-H bonds are different [8] and the infrared spectrum of ca. 93% deuterated urea nitrate at about 90 K shows four relatively sharp and strong bands (Fig. lc). The O-H bond is involved in a strong and almost linear hydrogen bridge, with an O-e-0 distance of 2.569 +-0.002 II. From the plot of O-H stretching frequency against 0 ---0 distance given by Novak [ 91, v(*H) should lie at quite a low frequency (- 2620 cm-‘), close to v(N-D). Figure 2 shows a plot of r(N-H) against v(N-H) (at ca. 90 K) for all the molecules studied. The closed circles refer to molecules for which the assignment of v(N-H) to a specific N-H bond is reasonably certain, normally either because the N-H bonds are identical [e.g., (HOOCH&NH;Brand K'SOxNH;] , or because they are very different (e.g., 2-amine-5-chloropyridine and 4-hydroxy-L-proline). The open circles are for urea and urea nitrate, where in view of the comparatively small differences in Y(N-H) and r(N-H), any assignments must be extremely tentative. The observed correlation is quite satisfactory, given the differences in hybridisation of the N-H bonds and the errors in the neutron diffraction data, and can be represented by the empirical relationship v(N-H) = 3477 (1 - 2.04 (r - 0.99) - 43.8 (r - 0.99)*). This diff ers from the linear relationship found by McKean [5] for CHDX2 compounds [rO= 1.402 - 0.0001035 u(C-H)] , presumably
149 3500
r, l
3400-
3300-
3200 T; 0 a
-
31003000-
2900
-
2800
-
2700
-
2600
, 0.98
I 0.99
I 1.00
I 1.01
I 1.02
I 1.03
I 1.04
0 I .05
r (8,
Fig. 2. Correlation between u(N-H) and r(N-H). (1) 2-amino-5chloropyridine; (2) Trimethoprim(2,4-diamino-5[3,4,5-trimethyoxybenyl J-pyrimidine); (a) two non-hydrogen bonded N-H, r = 0.991 f 0.002 A and 0.993 -c0.002 A, frequencies not resolved, mean value of 0.992 a taken for r; (b) two hydrogen bonded N-H, r for both is 1.020 f 0.002 A, frequencies not resolved; (3) (NH,),CO; (4) melamine. two non-hydrogen bonded N-H ( a ) in pyramidal C-NH, group (b) in almost planar C-NH,; (5) (NH,),COH+NO;; (6) NCNH,; (7) K+SO,NH;; (8) 4-hydroxy-L-proline; (9) (CH,),NH:HC,O;; (10) (HOOCCH,),NH;Br-. The curve represents the relationship u = 3477 (1 - 2.04 (r - 0.99) - 43.8 (r - 0.99)‘).
because r(N-H) is mainly determined by the hydrogen bonding effects. The deuterated compounds were prepared in the usuai way by exchange with D20, D,O/H,O or D20/CzHSOD. Infrared spectra were measured on Voltalef 3s mulls, using a Beckman RllC variable temperature cell and Perkin-Elmer 357 and 597 spectrometers. REFERENCES 1 A. Novak, J. Lascombe and M. L. Josien, J. Phys., 27 (1966) C-2. 2 A. Lautie, F. Froment and A. Novak, Spectrosc. Lett., 9 (1976) 289. 3 G. L. Hiebert and D. F. Homig, J. Lindgren and J. Tegenfeldt, J. Mol. Struct., 43 (1978) 179. 4 I. A. Oxton, 0. Knop and M. Falk, Can. J. Chem., 54 (1976) 892. 5 D. C. McKean, Chem. Sot. Rev., 7 (1978) 399. 6 Molecular Structures and Dimensions, 1935-76, International Union of Crystallography .bid., 1976-1977; I. Olousson and P.-G. Johnson in P. Schuster, G. Zundel and C. Sandorfy (Eds.), The Hydrogen Bond, North-Holland, Amsterdam, 1976, p. 393. 7 A, W. Pryor and P. L. Sanger, Acta Crystallogr., Sect. A, 26 (1970) 543. 8 J. E. Worshom and W. R. Busing, Acta Crystallogr., Sect. B, 25 (1969) 512. 9 A. Novak, Struct. Bonding (Berlin), 18 (1974) 177.
Journal of Molecular
Printed in The Netherlands
Short communication
“ISOLATED” CONTAINING
D. F. EVANS, Department
N-H STRETCHING NH2 GROUPS
FREQUENCIES
IN COMPOUNDS
P. H. MISSEN and M. W. UPTON
of Inorganic Chemistry,
Imperial College,
London
S?V7 2AY
(Gt. Britain)
(Received 8 December 1981)
Hydrogen bonding by N-H bonds is of some importance, not least in systems of biological interest. However, in many cases, the infrared spectra in the N-H stretching region exhibit very broad bands, even at low temperature (Fig. la)_ This limitation can often be overcome by extensive deuteration, so that isotopically dilute N-H bonds are present. In the absence of Fermi resonance, the number of isolated N-H stretches observabie should be equal to the number of chemically distinct N-H bonds. The isotopic dilution technique has been applied by Novak and his collaborators [ 1,2] (mainly at room temperature) to a number of compounds containing N-H bonds. Other applications.have been to solid hydrates [3] and ammonium salts [4] , while McKean
and his collaborators
[ 51 have made
extensive
studies
of compounds of the type CHD,X in the vapour phase. An excellent correlation between v(C-H) and r,-,(C-H) was observed. We have studied most molecules containing NH, groups, in the absence of NH or NH: groups, for which reasonably accurate neutron diffraction data are available [6 3 (standard deviation in r(N--H) < 0.006 a). The isolated N-H stretches observed at -90 K were found to correlate reasonably well with the chemically distinct N-H bonds present, and their internuclear distances_ The N-H stretches were distinguished from overtone or combination bands of the predominant fully deuterated species, u(N-H) of residual NH2 groups and C-H stretches, by varying the extent of deuteration in the approximate region W--97%. In urea, there are two pairs of equivalent
N-H
bonds.
HI \
N-Hz
0=C’ \ / HI
N-H2
These are involved in hydrogen bonds of very similar strength, and the N-H distances are indistinguishable within experimental error (0.998 + 0.005 A and 1.003 * 0.004 A) [7]. The infrared spectrum of -97% deuterated urea 0022-2860/82/0000-0000/$02.7
5 0 1982 Elsevier Scientific Publishing Company
148
80-
v (cm-‘)
60
201
3600
3400
3200
3000
u (cm-‘) Fig. 1. The infrared spectra at ca. 90 K of (a) urea, (b) deuterated (c) deuterated urea nitrate, protium ca. 7%.
urea, protium
ca. 3%
at ca. 90 K (Fig. lb) shows two strong sharp bands at 3367 cm-’ and 3394 bonds. cm-‘, which can be assigned to the two different types of N-H At room temperature, only a single, rather broad band was observed, as previously found by Lautie et al. [2]. In urea nitrate, (NH,),COWNO;, all four N-H bonds are different [8] and the infrared spectrum of ca. 93% deuterated urea nitrate at about 90 K shows four relatively sharp and strong bands (Fig. lc). The O-H bond is involved in a strong and almost linear hydrogen bridge, with an O-e-0 distance of 2.569 +-0.002 II. From the plot of O-H stretching frequency against 0 ---0 distance given by Novak [ 91, v(*H) should lie at quite a low frequency (- 2620 cm-‘), close to v(N-D). Figure 2 shows a plot of r(N-H) against v(N-H) (at ca. 90 K) for all the molecules studied. The closed circles refer to molecules for which the assignment of v(N-H) to a specific N-H bond is reasonably certain, normally either because the N-H bonds are identical [e.g., (HOOCH&NH;Brand K'SOxNH;] , or because they are very different (e.g., 2-amine-5-chloropyridine and 4-hydroxy-L-proline). The open circles are for urea and urea nitrate, where in view of the comparatively small differences in Y(N-H) and r(N-H), any assignments must be extremely tentative. The observed correlation is quite satisfactory, given the differences in hybridisation of the N-H bonds and the errors in the neutron diffraction data, and can be represented by the empirical relationship v(N-H) = 3477 (1 - 2.04 (r - 0.99) - 43.8 (r - 0.99)*). This diff ers from the linear relationship found by McKean [5] for CHDX2 compounds [rO= 1.402 - 0.0001035 u(C-H)] , presumably
149 3500
r, l
3400-
3300-
3200 T; 0 a
-
31003000-
2900
-
2800
-
2700
-
2600
, 0.98
I 0.99
I 1.00
I 1.01
I 1.02
I 1.03
I 1.04
0 I .05
r (8,
Fig. 2. Correlation between u(N-H) and r(N-H). (1) 2-amino-5chloropyridine; (2) Trimethoprim(2,4-diamino-5[3,4,5-trimethyoxybenyl J-pyrimidine); (a) two non-hydrogen bonded N-H, r = 0.991 f 0.002 A and 0.993 -c0.002 A, frequencies not resolved, mean value of 0.992 a taken for r; (b) two hydrogen bonded N-H, r for both is 1.020 f 0.002 A, frequencies not resolved; (3) (NH,),CO; (4) melamine. two non-hydrogen bonded N-H ( a ) in pyramidal C-NH, group (b) in almost planar C-NH,; (5) (NH,),COH+NO;; (6) NCNH,; (7) K+SO,NH;; (8) 4-hydroxy-L-proline; (9) (CH,),NH:HC,O;; (10) (HOOCCH,),NH;Br-. The curve represents the relationship u = 3477 (1 - 2.04 (r - 0.99) - 43.8 (r - 0.99)‘).
because r(N-H) is mainly determined by the hydrogen bonding effects. The deuterated compounds were prepared in the usuai way by exchange with D20, D,O/H,O or D20/CzHSOD. Infrared spectra were measured on Voltalef 3s mulls, using a Beckman RllC variable temperature cell and Perkin-Elmer 357 and 597 spectrometers. REFERENCES 1 A. Novak, J. Lascombe and M. L. Josien, J. Phys., 27 (1966) C-2. 2 A. Lautie, F. Froment and A. Novak, Spectrosc. Lett., 9 (1976) 289. 3 G. L. Hiebert and D. F. Homig, J. Lindgren and J. Tegenfeldt, J. Mol. Struct., 43 (1978) 179. 4 I. A. Oxton, 0. Knop and M. Falk, Can. J. Chem., 54 (1976) 892. 5 D. C. McKean, Chem. Sot. Rev., 7 (1978) 399. 6 Molecular Structures and Dimensions, 1935-76, International Union of Crystallography .bid., 1976-1977; I. Olousson and P.-G. Johnson in P. Schuster, G. Zundel and C. Sandorfy (Eds.), The Hydrogen Bond, North-Holland, Amsterdam, 1976, p. 393. 7 A, W. Pryor and P. L. Sanger, Acta Crystallogr., Sect. A, 26 (1970) 543. 8 J. E. Worshom and W. R. Busing, Acta Crystallogr., Sect. B, 25 (1969) 512. 9 A. Novak, Struct. Bonding (Berlin), 18 (1974) 177.