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Theoretical interpretation of the OH and NH stretching bands in the associated state showing submaxima in the diffuse absorption background
Research notes
1947
Spectrochimica Aeta, 1967, Vol. 23A, pp. 1947 to 1948. P e r g a m o n P r e s s L t d . P r i n t e d in N o r t h e r n I r e l a n d
Theoretical interpretation of the OH and NH stretching bands in the associated state showing submaYima in the diffuse absorption background (Received 14 September 1966) Abstxact--The fine structure of some OH and .NH chelate bands has been interpreted as being caused by a nearly harmonic "internal oscillation" of the hydrogen a t o m between two potential minima. B y deuteration, the fine structure becomes half as wide, corresponding to the theory. TH~ X H stretching band (vXH) of very strongly hydrogen bonded systems frequently lles at low frequencies, is rather diffuse, and either shows sub-maxima or cannot be identified. These types of bands have mainly been observed in the case of - - O H . . . O ~ hydrogen bonds but according to our investigations t h e y can also be found frequently in the case of N - - H . . . Y bonds. I n the case of strong intramolecular associates or cyclic dimers, the vOH and vNH bands show fine structure in the form of more or less equidistantly spaced submaxima. The following scheme shows the associate in which the proton is not in the centre of the bridged atoms: X--H...
X
X...
I
H--X II
I n such cases there are two equivalent equilibrium positions: I and II. The internal motion leading to the alternation of the two equilibrium positions, i.e., the "internal oscillation" of the proton is possible only when the energy barrier separating the two equilibria is not too high (smaller than t h e excitation energy of the X H stretching vibration). The function describing t h e spatial probability of finding the proton has two m a x i m a and therefore it is a good approximation to the distribution function of a harmonic oscillator for v = 1. I n such a case the proton undergoes a separate vibrational motion in the force field of the two heavier atoms. I n this state the square of the amplitude: X2 ~
hoJc
k
(1)
h 4#emHx ~
(2)
Hence: o(H)
According to our model in the process of the excitation of the internal vibration of the proton the frequency value calculated above and its multiples are superimposed upon the X H stretching frequency: o, =o(XH)
h ~ n - 4~CmHX2
(3)
The above equation gives the frequency of the sub-maxima, where n takes on small integral values. E q u a t i o n 3 is analogous to the formula giving the internal rotation structure of a vibrational band. I n agreement with equations 2 and 3 when substituting deuterium in the bridge the separation of submaxima drops to the half value. As it can be seen from equation 3 the separation of the submaxima is determined by the value of x (the distance between the potential minimum and the geometrical centre). The smaller is the value of x the higher is the frequency of the internal vibration and therefore the greater is the separation of the submaxima. When ~ is 0.95 J~ and the length of the H-bond is 2.90 A (~--~) (the data for ice) x = 0.5 A and the separation of the submaxima is 130 cm -1 in accordance with the experimental results. When the value for the ratio HX/X--Y is 0.5 the subm a x i m a disappear. When, in addition, the hydrogen bond is symmetrical (the X - - H bond
1948
R e s e a r c h notes
m o m e n t is small so t h e i n t e n s i t y of the K i t small also) the v X H b a n d itseff disappears (it merges into t h e base line due to t h e low i n t e n s i t y and diffuse structure).
Research Institute for Pharmaceutical Chemistry, Central Research Institute for Chemistry of the Hungarian Academy of Sciences, Budapest
P. SOHAR GY. V.~Rs£~,'YI
SpectrochlmicaActa, 1967, ¥ol. 23A, pp. 1948 to 1951. Pergamon Press Ltd. Printed in Northern Ireland
Nuclear magnetic resonance spectroscopy of selenomercaptans and selenols (Received 29 July 1966) A b s t r a c t - - T h e p r o t o n shift of S e t t group in selenomercaptans a n d selenols was studied. p - F l u o r o b e n z y l s e l e n o m e r c a p t a n a n d di-p-fluorobenzyl diselenide were p r e p a r e d for t h e first time. TH~ X~rHORS h a v e published r e c e n t l y t h e infra-red absorption spectra of selenomercaptans a n d selenols [I]. I n t h e present work, t h e p r o t o n shift of S e l l group f r o m t e t r a m e t h y l s i l a n e , as internal s t a n d a r d , was studied. All selenomercaptans a n d selenols were p r e p a r e d b y action of selenium powder, on a p p r o p r i a t e alkyl or a r y l m a g n e s i u m halides, u n d e r a n a t m o s p h e r e of hydrogen. p - F l u o r o b e n z y l s e l e n o m e r c a p t a n , b.p. 101°C/7 ram, n~SD 1-5352, yield 25 ~o a n d di-p-fluorobenzyl diselenide white plates f r o m 50 p e r cent alcohol, m.p. 83°C., yield 45 ~o as a b y p r o d u c t of t h e f o r m e r compound, were p r e p a r e d for t h e first time. A t t e m p t s to prepare t r i p h e n y l m e t h y l s e l e n o m e r e a p t a n a n d t - b u t y s e l e n o m e r c a p t a n failed. All selenomercaptans a n d solenols were distilled prior to spectroscopy. T h e c o m m o n s o l v e n t was carbon tetrachloride. T h e spectra were r e c o r d e d on a v a r i a n A 6 0 A s p e c t r o m e t e r . T h e o b s e r v e d p r o t o n shifts of SoH group in t h e i n v e s t i g a t e d selenomercaptans and selenols are s u m m a r i z e d in Table 1 and are expressed r e l a t i v e to t e t r a m e t h y l s i l a n o on t h e ~ scale. T a b l e 1. The observed p r o t o n shift of S e l l group (v) n-propylselenomercaptan n-butylselenomercaptan n-amylselenomercaptan Benzenselenol p -fluor0benzenselenol p-ehlorobenzenselenol u-Naphthylselenol
[2] [2] [6] [4] [ 1] [5] [4]
8"75 8"75 8"76 8"65 8"61 8"61 8"67
W e f o u n d t h a t the p r o t o n shift of SeI-[ group of b e n z y l s e l e n o m e r c a p t a n [3] was at r = 7.74 a n d t h e shift of p-fluorobenzylselenomereaptan was at z = 7.20. D i l u t i o n h a d no effect on p r o t o n shift of S o H group. [1] [2] [3] [4] [5] [6]
N. L. E. M. M. G.
S ~ m o H I a n d I. LALEZKRI, Spectrochim. Acta 20, 237 (1964). TSCKUGAYEW, Ber. 4,2, 49 (1909). P. PAINTER, J. Am. Chem. Soc. 69, 229 (1947). T~o~Y, Bull. Soc. Chin*. France (3) 29, 762 (1903). T~o~Y, ibid. (3) 35, 671 (1906). STOI~mR a n d R . WITZ.TAWS,J. Am. Chem. Soc. 70D 1113 (1948).
1947
Spectrochimica Aeta, 1967, Vol. 23A, pp. 1947 to 1948. P e r g a m o n P r e s s L t d . P r i n t e d in N o r t h e r n I r e l a n d
Theoretical interpretation of the OH and NH stretching bands in the associated state showing submaYima in the diffuse absorption background (Received 14 September 1966) Abstxact--The fine structure of some OH and .NH chelate bands has been interpreted as being caused by a nearly harmonic "internal oscillation" of the hydrogen a t o m between two potential minima. B y deuteration, the fine structure becomes half as wide, corresponding to the theory. TH~ X H stretching band (vXH) of very strongly hydrogen bonded systems frequently lles at low frequencies, is rather diffuse, and either shows sub-maxima or cannot be identified. These types of bands have mainly been observed in the case of - - O H . . . O ~ hydrogen bonds but according to our investigations t h e y can also be found frequently in the case of N - - H . . . Y bonds. I n the case of strong intramolecular associates or cyclic dimers, the vOH and vNH bands show fine structure in the form of more or less equidistantly spaced submaxima. The following scheme shows the associate in which the proton is not in the centre of the bridged atoms: X--H...
X
X...
I
H--X II
I n such cases there are two equivalent equilibrium positions: I and II. The internal motion leading to the alternation of the two equilibrium positions, i.e., the "internal oscillation" of the proton is possible only when the energy barrier separating the two equilibria is not too high (smaller than t h e excitation energy of the X H stretching vibration). The function describing t h e spatial probability of finding the proton has two m a x i m a and therefore it is a good approximation to the distribution function of a harmonic oscillator for v = 1. I n such a case the proton undergoes a separate vibrational motion in the force field of the two heavier atoms. I n this state the square of the amplitude: X2 ~
hoJc
k
(1)
h 4#emHx ~
(2)
Hence: o(H)
According to our model in the process of the excitation of the internal vibration of the proton the frequency value calculated above and its multiples are superimposed upon the X H stretching frequency: o, =o(XH)
h ~ n - 4~CmHX2
(3)
The above equation gives the frequency of the sub-maxima, where n takes on small integral values. E q u a t i o n 3 is analogous to the formula giving the internal rotation structure of a vibrational band. I n agreement with equations 2 and 3 when substituting deuterium in the bridge the separation of submaxima drops to the half value. As it can be seen from equation 3 the separation of the submaxima is determined by the value of x (the distance between the potential minimum and the geometrical centre). The smaller is the value of x the higher is the frequency of the internal vibration and therefore the greater is the separation of the submaxima. When ~ is 0.95 J~ and the length of the H-bond is 2.90 A (~--~) (the data for ice) x = 0.5 A and the separation of the submaxima is 130 cm -1 in accordance with the experimental results. When the value for the ratio HX/X--Y is 0.5 the subm a x i m a disappear. When, in addition, the hydrogen bond is symmetrical (the X - - H bond
1948
R e s e a r c h notes
m o m e n t is small so t h e i n t e n s i t y of the K i t small also) the v X H b a n d itseff disappears (it merges into t h e base line due to t h e low i n t e n s i t y and diffuse structure).
Research Institute for Pharmaceutical Chemistry, Central Research Institute for Chemistry of the Hungarian Academy of Sciences, Budapest
P. SOHAR GY. V.~Rs£~,'YI
SpectrochlmicaActa, 1967, ¥ol. 23A, pp. 1948 to 1951. Pergamon Press Ltd. Printed in Northern Ireland
Nuclear magnetic resonance spectroscopy of selenomercaptans and selenols (Received 29 July 1966) A b s t r a c t - - T h e p r o t o n shift of S e t t group in selenomercaptans a n d selenols was studied. p - F l u o r o b e n z y l s e l e n o m e r c a p t a n a n d di-p-fluorobenzyl diselenide were p r e p a r e d for t h e first time. TH~ X~rHORS h a v e published r e c e n t l y t h e infra-red absorption spectra of selenomercaptans a n d selenols [I]. I n t h e present work, t h e p r o t o n shift of S e l l group f r o m t e t r a m e t h y l s i l a n e , as internal s t a n d a r d , was studied. All selenomercaptans a n d selenols were p r e p a r e d b y action of selenium powder, on a p p r o p r i a t e alkyl or a r y l m a g n e s i u m halides, u n d e r a n a t m o s p h e r e of hydrogen. p - F l u o r o b e n z y l s e l e n o m e r c a p t a n , b.p. 101°C/7 ram, n~SD 1-5352, yield 25 ~o a n d di-p-fluorobenzyl diselenide white plates f r o m 50 p e r cent alcohol, m.p. 83°C., yield 45 ~o as a b y p r o d u c t of t h e f o r m e r compound, were p r e p a r e d for t h e first time. A t t e m p t s to prepare t r i p h e n y l m e t h y l s e l e n o m e r e a p t a n a n d t - b u t y s e l e n o m e r c a p t a n failed. All selenomercaptans a n d solenols were distilled prior to spectroscopy. T h e c o m m o n s o l v e n t was carbon tetrachloride. T h e spectra were r e c o r d e d on a v a r i a n A 6 0 A s p e c t r o m e t e r . T h e o b s e r v e d p r o t o n shifts of SoH group in t h e i n v e s t i g a t e d selenomercaptans and selenols are s u m m a r i z e d in Table 1 and are expressed r e l a t i v e to t e t r a m e t h y l s i l a n o on t h e ~ scale. T a b l e 1. The observed p r o t o n shift of S e l l group (v) n-propylselenomercaptan n-butylselenomercaptan n-amylselenomercaptan Benzenselenol p -fluor0benzenselenol p-ehlorobenzenselenol u-Naphthylselenol
[2] [2] [6] [4] [ 1] [5] [4]
8"75 8"75 8"76 8"65 8"61 8"61 8"67
W e f o u n d t h a t the p r o t o n shift of SeI-[ group of b e n z y l s e l e n o m e r c a p t a n [3] was at r = 7.74 a n d t h e shift of p-fluorobenzylselenomereaptan was at z = 7.20. D i l u t i o n h a d no effect on p r o t o n shift of S o H group. [1] [2] [3] [4] [5] [6]
N. L. E. M. M. G.
S ~ m o H I a n d I. LALEZKRI, Spectrochim. Acta 20, 237 (1964). TSCKUGAYEW, Ber. 4,2, 49 (1909). P. PAINTER, J. Am. Chem. Soc. 69, 229 (1947). T~o~Y, Bull. Soc. Chin*. France (3) 29, 762 (1903). T~o~Y, ibid. (3) 35, 671 (1906). STOI~mR a n d R . WITZ.TAWS,J. Am. Chem. Soc. 70D 1113 (1948).