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Intramolecular hydrogen bonds with large proton polarizability in semisalts of mono- and di-N-oxides of N,N′-tetraalkyl-o-xylyldiamines
Journal of Molecular Structure, 118 (1984) Elsevier Science
Publishers
B.V.,
Amsterdam
311-318 - Printed
in The Netherlands
INTRAMOLECWLAR HYDROGEN BONDS WITH LARGE PROTON POLARIZABILITY IN SEMISALTS OF MONO- and DI-N-OXIDES OF N,N’-TETRAALKYEo-XYLYLDIAMINES
BOGUMIL
BRZEZlNSKI
Institute of Chemistry A. Mickiewia GEORG
Uniuersify 60-780
Poznoii (PoZand)
ZUNDEL
Instifute of Physical Chemistry, (Received
13 February
University of Munich, D-8000
Munich 2 (West Germany)
1984)
ABSTRACT NO+H- . . ON t Heteroconjugated NO+H - -. N + NO - - . H+N and homoconjugated intramolecular hydrogen bonds formed in semisalts of monoand di-NNO. - - H’ON oxides of N,N’-tetraalkyl-o-xylyldiamines were studied by IR and NMR spectroscopy. All these hydrogen bonds show large proton polarizability. In the case of the heteroconjugated hydrogen bonds the proton transfer equilibrium shifts from compounds 1 to 3 to the left hand side since the interaction of the hydrogen bond with the solvent environment decreases in this series of compounds. With compound 1 the hydrogen bonds are slightly weaker and longer, hence the wavenumber dependence of the intensity of the continuum caused by these hydrogen bonds is slightly changed with compound 1 compared with compound 2. In the case of compound 3 the intensity of the continuum decreases because of increasing screening of the hydrogen bonds. In the series of homoconjugated hydrogen bonds, from compound 4 to 6 the intense continuum vanishes, and remains. The vanishing of the only the band of the O-l proton transition at 950 cmcontinuum is caused by increasing screening of the hydrogen bonds against their solvent enviromrents by bulky groups, and thus, this change demonstrates again that the interaction of the hydrogen bond with large proton polarizabilities is a necessary prerequisite for JR continua to appear.
RuR
0 cl-14t~~H 0 a a &R C”2 NkO I+
R”R ho
-
R=
0022-2860/84/$03.00
-5
CH3
1
&f-l7
2
C6H13
3
0
1984
,A
C’-‘$+ RR
Elsevier Science
Publishers
B.V.
312
a 0
RvR
CH,N-0 + CHZKo
B
x
0 a
RVR
‘V--9%.
H
CH,NOO’ RAR
RR R =
CH,
4
Cd47
5
C10’421
6
INTRODUCTION
Homoconjugated hydrogen bonds, for instance, B*H* * -B + B- - - H’B bonds with which the donor and the acceptor are the same type of group show large proton polarizabilities caused by proton motion within these bonds [l-3]. In heteroconjugated hydrogen bonds, AH - - - B =+ A- - - - H‘B, double minimum proton potentials may also be present whereby the interaction of these bonds with their environments is the main factor [4]. These proton transfer equilibria are shifted to the right hand side by these interaction effects [4, 51, whereby the interaction of the dipole of the hydrogen bond with the reaction field induced by it in its solvent environment is of main importance [5]. If the degree of asymmetry of these hydrogen bonds is not too great they also show large proton polarizability [5, 61. The strong interactions of the heteroconjugated hydrogen bonds with their solvent environments cause continua in the IR spectra [3--83, and conversely the presence of hydrogen bonds with large proton polarizability is indicated by such continua. These continua vanish and the usual bands are observed if these bonds are screened against their solvent environments by bulky groups [9-121. This prooves that the interaction of hydrogen bonds with large proton polarizability with their solvent environment is of decisive importance for the presence of the IRcontinua. The continua occur in different regions of the spectra and show different intensity distributions as a function of their mean length [3,13,14]. Short bonds show continua in the range 1500-800 cm-‘, medium long bonds continua extending over the whole region below 3000 cm-’ and long bonds cause continua with band-like structures in the region 2800-1600 cm-‘. Such continua have been studied already with a large number of intramolecular hydrogen bonds [lo, 141 (further refs. in ref. 10). In the following the influence of the hydrogen bond length and the influence of screening on the feature of the continua is studied with homoconjugated and heteroconjugated intramolecular hydrogen bonds in semisalts of mono and d&V-oxides of N,N’-tetraalkyl+xylyldiamines.
313 EXPERIMENTAL
General preparation procedure for mono-N-oxides. To a solution of 1 g base in 150 cm3 acetone, 0.2 cm3 30% l&O:, was added, and the mixture kept for 7-10 days at room temperature. The progress of the reaction was controlled by paper chromatography (disc technique) on Whatman No. 3 (diameter = 20 cm) with saturated aqueous ammonium sulphate as the developing phase. The chromatographic spots were made visible by means of iodine vapour. The excess of H302 was decomposed with 10% Pd-C. The mixture was allowed to stand overnight, filtered and the solution neutralized by means of a mixture of HC104 in EtOH (2%). After adding diethylether until the solution became cloudy, it was allowed to stand, yielding 0.98 g of white crystals. The crystals were recrystallized from ethanol with addition of diethylether. These perchlorates show correct analytical data. The preparation of the di-N-oxides and the analytical data are given in ref. 16. All spectra were recorded in CD&N solutions. CD&N was stored over a 3 8, molecular sieve. The NMR spectra were recorded with a VarianH4 100 spectrometer, calibrated against TMS as internal standard_ The IR spectra were obtained from 0.5 mol dmB3 CDBCN solutions, using a cell with NaCl windows (layer thickness 0.1 mm). The CD&N bands were compensated by using a cell with pure CD3CN in the reference beam. All IR spectra were recorded with a 325 spectrophotimeter (Bodenseewerk Perkin-Elmer, Uberlingen). RESULTS
AND
DISCUSSION
In Fig. la the IR spectra of the HAuCL,, semisalts of the N,N’-tetraalkyl+xylyldiamines, compound 1 (---, line), and compound 2 (-, line) are shown. The NMR data are given in Table 1. Strong hydrogen bonds are formed as shown by the large chemical shift towards lower fields of the hydrogenbonded proton. This shift is completely independent of the concentration, demonstrating that the bonds formed are intramolecular; (I) NO’H - - - N * NO - * - HN (II). Table 2 summarizes all experimental results for compounds l-3 _ In Fig. la the spectrum (solid line) shows that the semisalt of compound 2 causes an intense continuum in the region 1600-700 cm-‘. With compound 1 (dashed line) this continuum is less intense and this decrease is particularly pronounced at the low wavenumber side of the continuum. Hence the intensity distribution of the continuum shifts toward higher wavenumbers. In addition, a broad band-like structure arises in the region 2800-2100 cm-’ with maximum at about 2600 cm-‘. The continuum observed with compound 2 is characteristic for relatively short hydrogen bonds with large proton polarizability. From compound 2 to compound 1 the chemical shift of the hydrogen-bonded proton (Table 1) decreases from 18.86 ppm to 18.11 ppm, demonstrating that the (I>
Fig. 1. IR spectra of CD&N solutions. (a) ---, compound 1; -, compound 2; -a--, the free b&e of ccunpound 2. (b) -, compound 2; ---, compound 3; -*--, the free base of compound 2. TABLE 1 Chemical shifts (ppm) of the proton in the hydrogen bond in the H_4uCI, semisalts of the bases l-3 and in the HClO, semisalts of the bases 4-6 Compound
1 2 3 4 5 6
concentration (mol dm”) 0.1
0.2
0.4
0.5
0.8
1.0
18.11 18.86 18.82 18.55 18.52 18.50
18.11 18.86 18.82 18.55 18.52 7.8.50
18.11 18.85 18.83 ‘15.55 18.52 18.51
18.10 18.86 18.83 18.55 18.52 18.51
18.11 18.85 18.83 18.55 18.51 18.61
18.10 18.86 18.83 18.55 18.52 18.52
+ NO-- H’N (II) bonds become slightly weaker. This result is confirmed by the shift of the bending vibration of the hydrogen-bonded proton. This band is observed. for compound 2 at about 1720 cm-’ and for compound 1 at about 1630 cm-‘, indicating also a decrease in the strength of the hydrogen bonds. This change in the strength and hence also of the length of the NO’H - - * N =+ NO - - - H”N bonds explains the different spectral features of the continuum shown by compounds 2 and 1. In Fig. 2 calculated line spectra are shown as a function of the bond length and of the degree of asymmetry represented by the electrical field strength at the hydroge? bonds; see refs. 2 and 15. These line spectra suggest that with increasing mean length, the intensity distribution of the continuum shifts toward higher wavenumbers. Furthermore, these line spectra show that the NO+H---N
315 TABLE
2
Experimental
data for compounds Screening of the hydrogen bond
Compound
l-3
Chemical shift the hydrogenbonded oroton 18.11
1
2
6 OH cmy
Cantinu~~~ intensity and region in cm-
vNO*H at 930 cm-’
1630
Compared with 2, less intense and region shifted toward higher wa-,enumbers. band-like structure in the region 280&2100
o” El
1720
18.86
3
of
1720
18.83 1
o-o
t
1000
= 2.5
A
3000 5000 cm-l
intense 1600-700
in the region cm-
vN0 at 980 an4
5 E. iB
$ z
Intense band with mtimum n: 9 30 cmy
O-0 = 2.6 A
1000
3000 _, sooi) cm
side O-O distance 2.5 a, Fig. 2. Line spectra of the hydrogen bond in H,O?,, left-hand right-hand side O-O distance 2.6 A, as a function of the electrical field at the hydrogen bond (increasing asymmetry of the proton potential with increasing field strength) taken from ref. 15.
316
band-like structure in the region 2800-2100 cm-’ - observed with compound 1 - is probably mainly caused by the O-2 transition. Thus, the different intensity distributions of the continua as a function of the wavenumber are understandable since the (I) NO’H . *.N =+ NO --*WN (II) bonds are slightly weaker with compound 1 than with compound 2. With compound 1 the hydrogen bond, with large proton polarizability, is more easily accessible for the solvent. This stronger interaction weakens the hydrogen bond, and hence increases its length (ref. 3 pp. 723-724). Because of the different screenings of the hydrogen bonds against the solvent environments not only the bond length but also the position of the proton transfer equilibrium is changed. vNO is observed at about 930 cm-’ if the proton is present at this group, whereas this band is found at about 930 cm-’ in its absence. Thus, the change of the intensity of these bands (Fig. la) shows that from compound 2 to compound 1 the (I) NO+H - - - N =+ NO - - *H’N (II) equilibrium is slightly shifted to the right hand side. This shift of the equilibrium is caused by the increased interaction of the hydrogen bonds with large proton polarizability with their solvent environments [4-6,81In Fi.g. lb the spectra of the semisalts of compounds 2 and 3 are compared_ The band at 930 cm-’ is slightly more and the band at 980 cm-’ is slightly less intense with compound 3 than with compound 2. Hence with compound 3 the (I) NO+H- - -N =+ NO--- H+N (II) equilibrium is still less shifted to the left hand side than with compound 2. Thus with regard to the structure of the molecules this proton transfer equilibrium shifts from co.mpound 3 to 1, i.e., with decreasing screening of the hydrogen bonds by bulky groups it is shifted more and more to the right hand side since the interaction of the bonds with their environments increases. The continuum of compound 3 is much less intense than that of compound 2. The strengths of the hydrogen bonds are, however, the same, as shown by the same chemical shift of the hydrogen-bonded protons (Table 1) and by the fact that the band of the bending vibration of the proton in the hydrogen bonds is observed at 1720 cm-’ in both cases. The intensity decrease of the continuum with screening of the hydrogen bond demonstrates again that the interaction of the hydrogen bonds with their environments is a necessary prerequisite for the arising of the continua, a result which was already obtained with various systems [9-121, and will be confirmed in the following. Figure 3 shows the IR spectra of semisalts of compounds 4-6. The large chemical shifts of the hydrogen-bonded protons and their independence of concentration (Table 1) shows that strong intramolecular homoconjugated O’H--.O =+ 0. .-IT0 bonds are formed quantitatively. The fact that the size of the chemical shift of the hydrogen-bonded proton is the same in all three cases demonstrates that its bond strength does not change. In the series of compounds 4 to 6 the continuum vanished, however. With compound 6
317
Fig. 3. IR pound 6.
spectra
of
CD,CN
solutions,
-,
compound
4;
---,
compound
5;
---I,
com-
only an intense band at about 960 cm-’ is observed. This band is caused by the O-l transition (see Fig. 2, left hand side) of the proton in the hydrogen bond. The vanishing of the continuum with screening again shows, as already mentioned several times, that the interaction of the hydrogen bonds with large proton polarizability with their solvent environments is necessary for the occurrence of the continua. CONCLUSIONS
The semisalts of compounds l-3 form strong heteroconjugated ((I) NO’H---N =+ NO-- -H”N (II)) hydrogen bonds and those of compounds 4-6 strong homoconjugated intramolecular hydrogen bonds. With compounds l-3 both proton limiting structures have considerable weight, whereby the weight of proton limiting structure II decreases from compound 1 to compound 3. With compounds 1 and 2 IR continua indicate that these hydrogen bonds show large proton polarizability. The different wavenumber dependencies of these continua show that this bond is relatively short with compound 2 and slightiy longer with compound 1. Tlnis result is in agreement with the NMR data and with the shift of the band of the bending vibration of the proton in the hydrogen bond. The compolund 3 hydrogen bond has the same strength as that of compound 2. The intensity of the continuum is, however, much less since the (I) NO+H- --N + NO--- HN (II) bonds are screened against their solvent environments. With the homoconjugated NO’H- - - ON * NO - - - H’ON bonds in compounds 4 to 6 the continuum vanishes and with compound 6 only the intense band of the O-l proton transition remains at about 950 cm-‘. The continuum vanishes as a result of screening by bulky groups of the hydrogen bonds with large proton polarizability against their solvent environment. This result, in agreement with findings ‘obtained with ether compounds, demonstrates that the interaction of the hydrogen bonds with large proton polarizability with their solvent environments is a necessary prerequisite for the occurrence of IR continua.
318 ACKNOWLEDGEMENTS
Our thanks a~ due to the Deutsche Forschungsgemeinschaft and to the Fonds der Chemischen Industrie for providing the facilities for this work. REFERENCES 1 E. G. Weidemann and G. Zundel, Z. Naturforschung. Teil A, 25 (1970) 627. 2 R. Janoschek, E. G. Weidemann, H. Pfeiffer and G. Zundel, J. Am. Chem. Sot., 94 (1972) 2337. 3 G. Zundel, in P. Schuster, G. Zundel and C. Sandorfy (Eds.), The Hydrogen Bond Recent Developments in Theory and Experiments, Vol. II, Ch. 15, North-Holland, Amsterdam, 1976. 4 G. Zundel and J. Fritsch, J. Phys. Chem., in press. 5 J.Fritschuld G.Zundel,J.Phys.Chem. 85(1981)556. 6 R. Lindemann and G. Zundel, J. Chem. Sot. Faraday Trans. 2, 73 (1977)
788. 7 W. Boczofi, G. Pieczonka and M. Wiewibrowski, Tetrahedron, 33 (1977) 2665. 8 G. Zundel and J. Fritsch, in R. R. Dogonadze, E. Kam&n, A. A. Komyshev and J. Ulstrup (Eds.), Chemical Physics of Salvation. Vol. II, Elsevier, Amsterdam, 1985. 9 B. Brzezinski and G. Zundel, J. Chem. Sot. Faraday Trans., 76 (1980) 1061. 10 B. Brzezinski end G. Zundel, Can. J. Chem., 59 (1981) 786. 11 B. Brzezinski and G. Zundel, Chem. Phys. Lett., 87 (1982) 400. 12 B. Brzezinski and G. Zundel, Chem. Phys Lett., 95 (1983) 458. 13 A. Hayd, E. G. Weidemann and G. Zundel, J. Chem. Phys., 70 (1979) 86. 14 B. Brzezinski and G. Zundel, J. Mol. Struct., 84 (1982) 205. 15 R. Janoschek, E. G. Weidemann and G. Zundel, J. Chem. Sot. Faraday Trans. 2, 69 (1973) 505. 16 B. Brzezinski, Pol. J. Chem., (1983\.
Publishers
B.V.,
Amsterdam
311-318 - Printed
in The Netherlands
INTRAMOLECWLAR HYDROGEN BONDS WITH LARGE PROTON POLARIZABILITY IN SEMISALTS OF MONO- and DI-N-OXIDES OF N,N’-TETRAALKYEo-XYLYLDIAMINES
BOGUMIL
BRZEZlNSKI
Institute of Chemistry A. Mickiewia GEORG
Uniuersify 60-780
Poznoii (PoZand)
ZUNDEL
Instifute of Physical Chemistry, (Received
13 February
University of Munich, D-8000
Munich 2 (West Germany)
1984)
ABSTRACT NO+H- . . ON t Heteroconjugated NO+H - -. N + NO - - . H+N and homoconjugated intramolecular hydrogen bonds formed in semisalts of monoand di-NNO. - - H’ON oxides of N,N’-tetraalkyl-o-xylyldiamines were studied by IR and NMR spectroscopy. All these hydrogen bonds show large proton polarizability. In the case of the heteroconjugated hydrogen bonds the proton transfer equilibrium shifts from compounds 1 to 3 to the left hand side since the interaction of the hydrogen bond with the solvent environment decreases in this series of compounds. With compound 1 the hydrogen bonds are slightly weaker and longer, hence the wavenumber dependence of the intensity of the continuum caused by these hydrogen bonds is slightly changed with compound 1 compared with compound 2. In the case of compound 3 the intensity of the continuum decreases because of increasing screening of the hydrogen bonds. In the series of homoconjugated hydrogen bonds, from compound 4 to 6 the intense continuum vanishes, and remains. The vanishing of the only the band of the O-l proton transition at 950 cmcontinuum is caused by increasing screening of the hydrogen bonds against their solvent enviromrents by bulky groups, and thus, this change demonstrates again that the interaction of the hydrogen bond with large proton polarizabilities is a necessary prerequisite for JR continua to appear.
RuR
0 cl-14t~~H 0 a a &R C”2 NkO I+
R”R ho
-
R=
0022-2860/84/$03.00
-5
CH3
1
&f-l7
2
C6H13
3
0
1984
,A
C’-‘$+ RR
Elsevier Science
Publishers
B.V.
312
a 0
RvR
CH,N-0 + CHZKo
B
x
0 a
RVR
‘V--9%.
H
CH,NOO’ RAR
RR R =
CH,
4
Cd47
5
C10’421
6
INTRODUCTION
Homoconjugated hydrogen bonds, for instance, B*H* * -B + B- - - H’B bonds with which the donor and the acceptor are the same type of group show large proton polarizabilities caused by proton motion within these bonds [l-3]. In heteroconjugated hydrogen bonds, AH - - - B =+ A- - - - H‘B, double minimum proton potentials may also be present whereby the interaction of these bonds with their environments is the main factor [4]. These proton transfer equilibria are shifted to the right hand side by these interaction effects [4, 51, whereby the interaction of the dipole of the hydrogen bond with the reaction field induced by it in its solvent environment is of main importance [5]. If the degree of asymmetry of these hydrogen bonds is not too great they also show large proton polarizability [5, 61. The strong interactions of the heteroconjugated hydrogen bonds with their solvent environments cause continua in the IR spectra [3--83, and conversely the presence of hydrogen bonds with large proton polarizability is indicated by such continua. These continua vanish and the usual bands are observed if these bonds are screened against their solvent environments by bulky groups [9-121. This prooves that the interaction of hydrogen bonds with large proton polarizability with their solvent environment is of decisive importance for the presence of the IRcontinua. The continua occur in different regions of the spectra and show different intensity distributions as a function of their mean length [3,13,14]. Short bonds show continua in the range 1500-800 cm-‘, medium long bonds continua extending over the whole region below 3000 cm-’ and long bonds cause continua with band-like structures in the region 2800-1600 cm-‘. Such continua have been studied already with a large number of intramolecular hydrogen bonds [lo, 141 (further refs. in ref. 10). In the following the influence of the hydrogen bond length and the influence of screening on the feature of the continua is studied with homoconjugated and heteroconjugated intramolecular hydrogen bonds in semisalts of mono and d&V-oxides of N,N’-tetraalkyl+xylyldiamines.
313 EXPERIMENTAL
General preparation procedure for mono-N-oxides. To a solution of 1 g base in 150 cm3 acetone, 0.2 cm3 30% l&O:, was added, and the mixture kept for 7-10 days at room temperature. The progress of the reaction was controlled by paper chromatography (disc technique) on Whatman No. 3 (diameter = 20 cm) with saturated aqueous ammonium sulphate as the developing phase. The chromatographic spots were made visible by means of iodine vapour. The excess of H302 was decomposed with 10% Pd-C. The mixture was allowed to stand overnight, filtered and the solution neutralized by means of a mixture of HC104 in EtOH (2%). After adding diethylether until the solution became cloudy, it was allowed to stand, yielding 0.98 g of white crystals. The crystals were recrystallized from ethanol with addition of diethylether. These perchlorates show correct analytical data. The preparation of the di-N-oxides and the analytical data are given in ref. 16. All spectra were recorded in CD&N solutions. CD&N was stored over a 3 8, molecular sieve. The NMR spectra were recorded with a VarianH4 100 spectrometer, calibrated against TMS as internal standard_ The IR spectra were obtained from 0.5 mol dmB3 CDBCN solutions, using a cell with NaCl windows (layer thickness 0.1 mm). The CD&N bands were compensated by using a cell with pure CD3CN in the reference beam. All IR spectra were recorded with a 325 spectrophotimeter (Bodenseewerk Perkin-Elmer, Uberlingen). RESULTS
AND
DISCUSSION
In Fig. la the IR spectra of the HAuCL,, semisalts of the N,N’-tetraalkyl+xylyldiamines, compound 1 (---, line), and compound 2 (-, line) are shown. The NMR data are given in Table 1. Strong hydrogen bonds are formed as shown by the large chemical shift towards lower fields of the hydrogenbonded proton. This shift is completely independent of the concentration, demonstrating that the bonds formed are intramolecular; (I) NO’H - - - N * NO - * - HN (II). Table 2 summarizes all experimental results for compounds l-3 _ In Fig. la the spectrum (solid line) shows that the semisalt of compound 2 causes an intense continuum in the region 1600-700 cm-‘. With compound 1 (dashed line) this continuum is less intense and this decrease is particularly pronounced at the low wavenumber side of the continuum. Hence the intensity distribution of the continuum shifts toward higher wavenumbers. In addition, a broad band-like structure arises in the region 2800-2100 cm-’ with maximum at about 2600 cm-‘. The continuum observed with compound 2 is characteristic for relatively short hydrogen bonds with large proton polarizability. From compound 2 to compound 1 the chemical shift of the hydrogen-bonded proton (Table 1) decreases from 18.86 ppm to 18.11 ppm, demonstrating that the (I>
Fig. 1. IR spectra of CD&N solutions. (a) ---, compound 1; -, compound 2; -a--, the free b&e of ccunpound 2. (b) -, compound 2; ---, compound 3; -*--, the free base of compound 2. TABLE 1 Chemical shifts (ppm) of the proton in the hydrogen bond in the H_4uCI, semisalts of the bases l-3 and in the HClO, semisalts of the bases 4-6 Compound
1 2 3 4 5 6
concentration (mol dm”) 0.1
0.2
0.4
0.5
0.8
1.0
18.11 18.86 18.82 18.55 18.52 18.50
18.11 18.86 18.82 18.55 18.52 7.8.50
18.11 18.85 18.83 ‘15.55 18.52 18.51
18.10 18.86 18.83 18.55 18.52 18.51
18.11 18.85 18.83 18.55 18.51 18.61
18.10 18.86 18.83 18.55 18.52 18.52
+ NO-- H’N (II) bonds become slightly weaker. This result is confirmed by the shift of the bending vibration of the hydrogen-bonded proton. This band is observed. for compound 2 at about 1720 cm-’ and for compound 1 at about 1630 cm-‘, indicating also a decrease in the strength of the hydrogen bonds. This change in the strength and hence also of the length of the NO’H - - * N =+ NO - - - H”N bonds explains the different spectral features of the continuum shown by compounds 2 and 1. In Fig. 2 calculated line spectra are shown as a function of the bond length and of the degree of asymmetry represented by the electrical field strength at the hydroge? bonds; see refs. 2 and 15. These line spectra suggest that with increasing mean length, the intensity distribution of the continuum shifts toward higher wavenumbers. Furthermore, these line spectra show that the NO+H---N
315 TABLE
2
Experimental
data for compounds Screening of the hydrogen bond
Compound
l-3
Chemical shift the hydrogenbonded oroton 18.11
1
2
6 OH cmy
Cantinu~~~ intensity and region in cm-
vNO*H at 930 cm-’
1630
Compared with 2, less intense and region shifted toward higher wa-,enumbers. band-like structure in the region 280&2100
o” El
1720
18.86
3
of
1720
18.83 1
o-o
t
1000
= 2.5
A
3000 5000 cm-l
intense 1600-700
in the region cm-
vN0 at 980 an4
5 E. iB
$ z
Intense band with mtimum n: 9 30 cmy
O-0 = 2.6 A
1000
3000 _, sooi) cm
side O-O distance 2.5 a, Fig. 2. Line spectra of the hydrogen bond in H,O?,, left-hand right-hand side O-O distance 2.6 A, as a function of the electrical field at the hydrogen bond (increasing asymmetry of the proton potential with increasing field strength) taken from ref. 15.
316
band-like structure in the region 2800-2100 cm-’ - observed with compound 1 - is probably mainly caused by the O-2 transition. Thus, the different intensity distributions of the continua as a function of the wavenumber are understandable since the (I) NO’H . *.N =+ NO --*WN (II) bonds are slightly weaker with compound 1 than with compound 2. With compound 1 the hydrogen bond, with large proton polarizability, is more easily accessible for the solvent. This stronger interaction weakens the hydrogen bond, and hence increases its length (ref. 3 pp. 723-724). Because of the different screenings of the hydrogen bonds against the solvent environments not only the bond length but also the position of the proton transfer equilibrium is changed. vNO is observed at about 930 cm-’ if the proton is present at this group, whereas this band is found at about 930 cm-’ in its absence. Thus, the change of the intensity of these bands (Fig. la) shows that from compound 2 to compound 1 the (I) NO+H - - - N =+ NO - - *H’N (II) equilibrium is slightly shifted to the right hand side. This shift of the equilibrium is caused by the increased interaction of the hydrogen bonds with large proton polarizability with their solvent environments [4-6,81In Fi.g. lb the spectra of the semisalts of compounds 2 and 3 are compared_ The band at 930 cm-’ is slightly more and the band at 980 cm-’ is slightly less intense with compound 3 than with compound 2. Hence with compound 3 the (I) NO+H- - -N =+ NO--- H+N (II) equilibrium is still less shifted to the left hand side than with compound 2. Thus with regard to the structure of the molecules this proton transfer equilibrium shifts from co.mpound 3 to 1, i.e., with decreasing screening of the hydrogen bonds by bulky groups it is shifted more and more to the right hand side since the interaction of the bonds with their environments increases. The continuum of compound 3 is much less intense than that of compound 2. The strengths of the hydrogen bonds are, however, the same, as shown by the same chemical shift of the hydrogen-bonded protons (Table 1) and by the fact that the band of the bending vibration of the proton in the hydrogen bonds is observed at 1720 cm-’ in both cases. The intensity decrease of the continuum with screening of the hydrogen bond demonstrates again that the interaction of the hydrogen bonds with their environments is a necessary prerequisite for the arising of the continua, a result which was already obtained with various systems [9-121, and will be confirmed in the following. Figure 3 shows the IR spectra of semisalts of compounds 4-6. The large chemical shifts of the hydrogen-bonded protons and their independence of concentration (Table 1) shows that strong intramolecular homoconjugated O’H--.O =+ 0. .-IT0 bonds are formed quantitatively. The fact that the size of the chemical shift of the hydrogen-bonded proton is the same in all three cases demonstrates that its bond strength does not change. In the series of compounds 4 to 6 the continuum vanished, however. With compound 6
317
Fig. 3. IR pound 6.
spectra
of
CD,CN
solutions,
-,
compound
4;
---,
compound
5;
---I,
com-
only an intense band at about 960 cm-’ is observed. This band is caused by the O-l transition (see Fig. 2, left hand side) of the proton in the hydrogen bond. The vanishing of the continuum with screening again shows, as already mentioned several times, that the interaction of the hydrogen bonds with large proton polarizability with their solvent environments is necessary for the occurrence of the continua. CONCLUSIONS
The semisalts of compounds l-3 form strong heteroconjugated ((I) NO’H---N =+ NO-- -H”N (II)) hydrogen bonds and those of compounds 4-6 strong homoconjugated intramolecular hydrogen bonds. With compounds l-3 both proton limiting structures have considerable weight, whereby the weight of proton limiting structure II decreases from compound 1 to compound 3. With compounds 1 and 2 IR continua indicate that these hydrogen bonds show large proton polarizability. The different wavenumber dependencies of these continua show that this bond is relatively short with compound 2 and slightiy longer with compound 1. Tlnis result is in agreement with the NMR data and with the shift of the band of the bending vibration of the proton in the hydrogen bond. The compolund 3 hydrogen bond has the same strength as that of compound 2. The intensity of the continuum is, however, much less since the (I) NO+H- --N + NO--- HN (II) bonds are screened against their solvent environments. With the homoconjugated NO’H- - - ON * NO - - - H’ON bonds in compounds 4 to 6 the continuum vanishes and with compound 6 only the intense band of the O-l proton transition remains at about 950 cm-‘. The continuum vanishes as a result of screening by bulky groups of the hydrogen bonds with large proton polarizability against their solvent environment. This result, in agreement with findings ‘obtained with ether compounds, demonstrates that the interaction of the hydrogen bonds with large proton polarizability with their solvent environments is a necessary prerequisite for the occurrence of IR continua.
318 ACKNOWLEDGEMENTS
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