- Email: [email protected]
Microwave spectrum of salicyl aldehyde: Structure of the hydrogen bond
aJOURNaL
OF
MOLECULAR
SPECTROSCOPY
Microwave
42, 65-74
(1972)
Spectrum of Salicyl Aldehyde: of the Hydrogen Bond
Structure
HAROLD JONES~AND R. F. CURL, JR. Department of Chemistry,
Rice University,
Houston,
Texas YYOOi
The microwave spectrum of salicyl aldehyde and two isotopic modifications, OD and -CHW, have been observed and assigned. The molecule is planar and the distance from the hydroxy proton to the aldehyde oxygen is 1.76 A. In contrast to previously studied 2-aminoethanol, no structural alterations due to the presence of the hydrogen bond are observed. INTRODUCTION
Several systems containing intramolecular hydrogen bonds have been studied by means of microwave spectroscopy. These include glycolaldehyde (I), and the 2 substituted ethanols, 2-fluoroethanol (Z), 2-chloroethanol (S), 2bromoethanol (S), and 2-aminoethanol (4). The studies of the 2 substituted ethanols indicate that the OH bond distance is significantly lengthened by hydrogen bonding. In these systems this lengthening is linearly related to the shift in the OH stretching frequency with the proportionality constant 0.0016 A/cm-l. If the shift in the OH stretching frequency is used as measure of hydrogen bond strength, then a number of systems may be found with much stronger hydrogen bonds than those already studied One such syst.em is salicyl aldehyde with a frequency shift of about 430 cm-l (5) as compared to 115 cm-’ for 2-aminoethanol. Extrapolating the linear relationship of OH bond length to infrared shift, the projected OH bond length for salicyl aldehyde would be ~1.5 A! Of course, it was thought most unlikely that such an extrapolation would be valid. However, it seemed likely that some marked effect on structure due to the very strong hydrogen bond would be found in salicyl aldehyde. EXPERIMENTAL
Normal salicyl aldehyde was purchased from Matheson, Coleman and Bell and was used without further purification. A sample of salicyl aldehyde in which the hydroxyl proton had been replaced by deuterium was produced by direct exchange with excess deuterium oxide. The 1Present address: Physical Chemistry Depart#ment, Cambridge University, Cambridge, England. 65 Copyright
@ 1972 by Academic
Press, Inc.
66
JONES AND CURL
MICROWAVE
SPECTRUM
OF SALICYL
TABLE OBSERVED
TRANSITIONS Transition
%,9
c102,8
113,9 +l”3 ,8 113,8 +lO3,7 114,8 +“4,7
ALDEHYDE
I
OF NORMAL
SALICYL ALDEHYDE
Observed 28077.62
-0.09
27265.10
0.14
30049.48
0.11
28301.80
0.13
29869.30
0.23
+l”4,6
?,7
+105,6
28499.42
-0.22
‘l5,6 *105,5
28805.12
0.12
‘=6,6 +l”6,5
28353.66
0.03
116,5 +l”6,4
28378.12
0.02
%,3
t1O8,2
28113.12
0.00
ll9.2 +109,1
28048.06
0 .Ol
121,11 +111,10
27589.58
0.04
122,11 +112,10
27541.04
0.18
“6,6
31101.86
0.02
‘*6,7 +116,6
31036.18
0.02
ll17,5
30858.53
0.11
127,5 +117,4
30861.86
-0.32
‘*8,4 +%,3
30732.77
-0.12
129,3 +119,2
30647.75
-0.04
1210,2 clllo,l
30587.67
-0.15
1211,1 tllll,O
30543.60
-0.15
130,13 c~*o,12
27544.42
0.05
29612.20
-0.04
29584.12
-0.04
31624.12
-0 .oo
“7,6
%,12
c121,11
l40,14 +130,13 l50,15 +140,14
(MHz)
*.-a.
=4,7
+116,5
67
oxygen-18 content of the carboxyl group of s,alicyl aldehyde was increased to approximately 40 % by exchange with excess acidified 40 % oxygen-18-enriched water. Both exchange reactions occurred readily in the liquid phase at room temperature. The salicyl aldehyde was separated from the water by vacuum distillation. The microwave spectra were recorded on a Hewlett-Packard model 8460A microwave spectrometer, within the range 26.5-40.0 GHz. The spectrum of the normal species is shown in Fig. 1.
JONES AND CURL TABLE II OBSERVED
TRANSITIONS-O-D
Tram ition
SALICYL ALDEHYDE
(MHz)
Observed
Obs-Calc
lL2,9 c102,8
27793.04
-0.12
ll3,v +lO3,8
26986.78
0.02
_lO3,7
29741.17
-0.06
114,8 *104,7
28011.26
0.05
114,7 +104,6
29559.14
0.08
%,7
+105,6
28206.20
0.13
%,6
+lO5,5
28507.05
0.15
ll6,5 +106,4
23085.65
0.10
116,6 +l"6,5
28061.62
0.14
117,4 +lO7,3
27919.85
-0.17
118,3*108,2
27823.80
0.00
119,2cLo9,1
27759.80
0.30
121,11elll,10
27309.33
0.04
122,11+112,10
27261.73
0.08
122,1Ll +112,9
29677.50
0.30
123,10+113,9
29142.93
0.01
123,9cL13,8
32091.51
-0.10
124,V +ll4,8
30430.19
0.05
124,8+l14,7
32559.42
0.10
"5,8 *115,7
30830.20
0.01
125,7+"5,6
31434.18
0.08
=6,7 +116,6
30716.32
0.06
126,6+116,5
30780.64
-0.22
127,6+'l7,5
30540.60
0.07
127,5 +117,4
30543.92
-0.32
128 4 +11 893
30416.36
-0.15
129,3 +119,2
30332.22
-0.19
1210,2tlllO,l
30272.90
-0.27
13 0,13 t120,12
27264.45
0.03
131,12c121,11
29311.22
-0.15
13 2,12 c122,11
29288.40
0.03
14 0,14 *l30,13
29283.46
-0.02
%,8
OBSERVED
TUNSITIONS
Transition
112,9cL02,8 LL3,8 *L"3,7 'L4,8*104,7 LL5,7*LO5,6 LL5,6+L05,5 LL6,6'L06,5 'L6,5+l"6>4 117,5*LO7,4 L18,4 +"8,3
TABLE: III -CHOl* S~LICYL ALDEHYDE
(MHz)
Observed
Obs-Calc
27350.00
0.26
29004.30
0.10
27314.29
-0.04
27435.01
0.07
27645.38
0.17
27291.92
-0.14
27306.90
-0.20
27167.05
0.05
27084.56
-0.08
27028.82
-0.20
26837.78
0.16
26770.50
0.00
29222.98
0.31
28566.20
-0.09
29707.36
0.23
31532.40
0.14
29906.45
-0.07
29865.80
0.02
29709.10
0.12
29601.25
-0.31
29528.65
-0.25
26779.52
-0.32
119,3+LOv,z 1*1,11+LLl,lO 122,ll+Ll2,10 122,lO+L12,9 12 3,lO +113,9 124,9*L14,8 124,8*"4,7 126 6 +'L6 5 126;7+116:6 127 6+117 5 "8,4 +118,3 129>3 +LL9,2 13 0,13+L20,12 131,12 +L21,11 i32,llCL22,lO L32,12+L22,11 140,14+L30,13 L42,13+132,12 142,12 +132,11 150>15 +140t14 L51,14*141,13 152,14'142,13 160,16c150,15 L70,17+L60,16
l
180,18 170,17
28798.75
-0.02
31074.65
-0.21
28764.92
0.07
28162.90
0.04
30752.94
0.04
32949.98
-0.26
30746.U
0.04
32745.38
-0.36
32737.72
-0.04
32729.62
0.19
34713.00
0.17
36696.38
0.11
69
JONES AND CURL
70
ANALYSIS OF THE SPECTRUM
Molecular model calculations indicated that salicyl aldehyde was a prolate asymmetric rotor with rotational constants of approximately A = 3380, B = 1,525, C = 1051 MHz, and Ray’s asymmetry parameter K = -0.59. The model also indicated that the a component of the dipole moment was the largest. Since the rotational constants were quite small, the a-type transitions which fell into the region 26.5-40.0 GHz had quite high values of the rotational quantum number J. As a result of this, little aid was gained from Stark effects. As may be seen from Fig. 1 u-type R-branch bands were prominent in the spectrum, and little difficult’y was experienced in identifying individual transitions within t,hese bands. Table I shows a least square fit of 25 lines for the normal species of salicyl aldehyde. Tables II and III show least square fits for 32 lines of C6H4. CHO.OD and 37 lines of CsH4. CHl*O. OH, respectively. In all three cases, the fit to a rigid rotor model was good and there appears to be no significant centrifugal distortion effect’s present. This is in keeping wiOh observations on nitrobenzene (6). Only a-type, R-branch transitions were measured for all three isotopic forms of salicyl aldehyde. However, the molecule was sufficiently asymmetric to allow accurate values of all three rotat’ional constants to be determined. Table IV TABLE IV ROT~TION.&L
CONST.INTS OF S.~I,ICYLALDEHYDE E!
A
(MHz)
c
Model
3249
1525
1038
Normal
3215.97
1493.618
1020.110
-0-D
3184.44
1478.184
1009.742
-CHOl8
3212.01
1434.501
991.779
Standard deviation in observed
0.06
0.004
0.004
TABLE V MOLECULAR
MOMENTS
OF INERTIA OF S~LICYL ALDEHYDE(AMU
AZ)
f Normal
157.194
338.461
495.565
-0.091
-0-D
158.750
341.994
500.653
-0.091
-MO18
157.387
352.409
509.721
-0.075
(conversion
factor 5.05531 x 105mi+.m.u.
A21
MICROWAVE
SPECTRUM
OF SALICYL
TABLE COORDIXATES Experimental
ALDEHYDE:
71
VI CA) a
b
Deuterium atom
+1.a73
-1.260
018 atom
+2.657
+0.32&
Model Deuterim atw
+1.97
-1.29
P
+2.60
40.33
atmn
shows the rotationa constants for all three isotopic species. The error limit quoted represents the standard deviation obtained from the least-square fits of the observed transitions. As expected, the rotational constant A was determined with less certainty than the other two constants. MOLECULAR
STRUCTURE
The molecular moments of inertia calculated from the observed rotational constants are shown in Table V. For a planar molecule the inertial defect, I,-I,-Ib , should be zero, but usually vibrational averaging effects cause the inertial defect to become nonzero. As can be seen in Table V, the inertial defect of all three isotopic species of salicyl aldehyde is very small and negative. The fact that the inertial defect is negative shows that out-of-plane vibrational motions predominate, these motions probably being the aldehyde and phenol group torsions. The magnitude of the defect, approximately -0.085 amu AZ, fits in well with observations on fluorobenzene (A = +0.03) (7) and nitrobenzene (A = -0.48) (6). There can be no doubt that salicyl aldehyde is planar. With the data available it was possible to locate the phenolic proton and the carboxyl group oxygen by use of Kraitchman’s equations (8). The resulting calculated coordinates are shown in Table VI. There seems to be little ambiguity about the sign of the coordinates since the signs shown place the two atoms extremely close to the positions indicated by the initial molecular model. The molecular model used was based on an X-ray diffraction study of salicylic acid (9). Dimensions used for the aldehyde group were those observed in formaldehyde (10). It was found that if the C-O bond of the phenol group was tilted so that it made an angle of 6” with a line bisecting the ring angle at that point, a model was produced which not only reproduced the observed rotational constants well (Table V) but also placed the hydrogen and oxygen atoms extremely close to the coordinates calculated from Braitchman’s equations (Table VI). A diagram of the molecular geometry is given in Fig. 2. This angle of tilt is in keeping with the X-ray studies on the potassium salt of o-nitrophenol (11) in which a similar angle (6’) was observed. Thus it would seem that an extremely long O-H bond is not present. The obvious way to show this conclusively is to study a sample in
72
JONES
AND
CURL
FIG. 2. Molecular model which fairly closely reproduces the normal species rotational constants and the principal axis coordinates of the phenol hydrogen and aldehyde oxygen. The coordinates determined for these at’oms from Kraitchman’s equations are indicated by X.
which the phenolic oxygen has been enriched with oxygen-l% This possibility was considered closely but subsequently abandoned because of the lack of an economically feasible synthetic route. The quantity determined directly in this study is the nonbonded (or hydrogen bonded) distance between th,e phenolic proton and the carbonyl group proton, this value being 1.76 f 0.01 A. The error quoted here reflects only the estimated experimental errors; no attempt has been made to allow for such effects as zeropoint vibration. This distance is considerably less than the sum of the van der Waal’s radii of hydrogen and oxygen, and in the absence of some stabilisation extremely large steric repulsions would be expected. DISCUSSION
Thus we have a situation attributed to the moderately
in which significant structural changes weak hydrogen bond of Zaminoethanol
have been (4) while
MICROWAVE
SPECTRUM OF SALICYL ALDEHYDE
73
0.
P-AMINOETHANOL
-10
r
SALICYL
ALOEHYDE
FIG. 3. Energy versus H.*.B distance for systems which have strong hydrogen bonds. H is the hydrogen bonding hydrogen and B is the base (in t,he case of salicyl aldehyde 0). The two points marked correspond to salicyl aldehyde and 2-aminoethanol. The fact that 2-aminoethanol has an H. . . B distance which corresponds to a rapid change of energy with distance indicates that there is a strong force attracting the H to the base (N) in %-aminoethanol. This strong attractive force may account for the observed bond lengthening. In the case of salioyl aldehyde the rate of change of energy with H. . .B distance is small, little attractive force is felt, and the 0.. +H distance is normal. Note that the hydrogen bonding energy of salicyl aldehyde is considerably greater than that of 2-aminoethanol according to the figure.
there appears to be no unusual feature of the structure of the strongly hydrogen bonding salicyl aldehyde. We rationalize this situation by the following arguments. Presumably, as the hydrogen bonding hydrogen (in these cases a hydroxy group) is brought up from infinity to the base, the energy decreases to a minimum and then increases. The minimum point corresponds to the observed structure of intermolecular hydrogen bonding systems and for strongly hydroge? bonding systems of the 0-H. . .O type is an 0. . .O distance of about 2.5-2.6 A (2.64 A in propionic acid dimer) (12) or to an H **.O dist’ance of about 1.6 A. This behavior is illustrated in Fig. 3. In 2-aminoethanol the molecular framework prevents the hydroxy hydrogen from coming closer to the nitrogen atom. Thus the hydrogen bonding forces are primarily attractive and these attractive forces account for the bond lengthening of 2-aminoethanol. In the case of salicyl aldehyde the H +. a0 distance is near the equilibrium point of intermolecular systems and the attractive and repulsive forces are nearly balanced. ACKNOWLEDGMENTS Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. This work was also supported by Grant C-071 of The Robert A. Welch Foundation.
74
JONES
A.ND CURL
RECEIVED : September ZO,l971 REFERENCES 1. K. M. M~RSTOKE~AND H. MOELLENDAL,J. iVoZ. Struct. 6,205 (1970). 2. K. S. BUCKTONAND R. G. AZRAK, J. Chem. Phys. 62, 5652 (1970). 3. R. G. AZRAK AND E. B. WILSON, J. Chem. Phys. 52,5299 (1970). $. R. E. PENN .~NDR. F. CURL, J. Chem. Phys. 66,651 (1971). 5. D. HEINERT AND A. E. MBTELL, J. Amer. Chew Sot. 81,3933 (1959). 6. J. H. HOG, L. NYGAARD, AND G. 0. SORENSEN,1. Mol. Struct. 7, 111 (1971). 7. L. NYGAARD, I. BOJESON,T. PEDERSEN,BNDJ. R.~RTRUP-ANDERSEN, J. Mol. Struct. 2, 209 (1968). 8. J. KRAITCHMAN,Amer. J. Phys. 21,17 (1953). 9. W. COcHR.IN, Acta Crystdogr. 6, 260 (1953). 10. T. OKA, J. Phys. Sot. Jap. 16,2274 (1960). If. J. P. G. RICHARDS,2. Kristallagr. 116,468 (1961). 12. F. J. STRIETER,D. H. TEMPLETON,R. F. SCHEUERM~N,.\NDIt. L. SASS, Acta Crystallogr. 16, 1233 (1962).
OF
MOLECULAR
SPECTROSCOPY
Microwave
42, 65-74
(1972)
Spectrum of Salicyl Aldehyde: of the Hydrogen Bond
Structure
HAROLD JONES~AND R. F. CURL, JR. Department of Chemistry,
Rice University,
Houston,
Texas YYOOi
The microwave spectrum of salicyl aldehyde and two isotopic modifications, OD and -CHW, have been observed and assigned. The molecule is planar and the distance from the hydroxy proton to the aldehyde oxygen is 1.76 A. In contrast to previously studied 2-aminoethanol, no structural alterations due to the presence of the hydrogen bond are observed. INTRODUCTION
Several systems containing intramolecular hydrogen bonds have been studied by means of microwave spectroscopy. These include glycolaldehyde (I), and the 2 substituted ethanols, 2-fluoroethanol (Z), 2-chloroethanol (S), 2bromoethanol (S), and 2-aminoethanol (4). The studies of the 2 substituted ethanols indicate that the OH bond distance is significantly lengthened by hydrogen bonding. In these systems this lengthening is linearly related to the shift in the OH stretching frequency with the proportionality constant 0.0016 A/cm-l. If the shift in the OH stretching frequency is used as measure of hydrogen bond strength, then a number of systems may be found with much stronger hydrogen bonds than those already studied One such syst.em is salicyl aldehyde with a frequency shift of about 430 cm-l (5) as compared to 115 cm-’ for 2-aminoethanol. Extrapolating the linear relationship of OH bond length to infrared shift, the projected OH bond length for salicyl aldehyde would be ~1.5 A! Of course, it was thought most unlikely that such an extrapolation would be valid. However, it seemed likely that some marked effect on structure due to the very strong hydrogen bond would be found in salicyl aldehyde. EXPERIMENTAL
Normal salicyl aldehyde was purchased from Matheson, Coleman and Bell and was used without further purification. A sample of salicyl aldehyde in which the hydroxyl proton had been replaced by deuterium was produced by direct exchange with excess deuterium oxide. The 1Present address: Physical Chemistry Depart#ment, Cambridge University, Cambridge, England. 65 Copyright
@ 1972 by Academic
Press, Inc.
66
JONES AND CURL
MICROWAVE
SPECTRUM
OF SALICYL
TABLE OBSERVED
TRANSITIONS Transition
%,9
c102,8
113,9 +l”3 ,8 113,8 +lO3,7 114,8 +“4,7
ALDEHYDE
I
OF NORMAL
SALICYL ALDEHYDE
Observed 28077.62
-0.09
27265.10
0.14
30049.48
0.11
28301.80
0.13
29869.30
0.23
+l”4,6
?,7
+105,6
28499.42
-0.22
‘l5,6 *105,5
28805.12
0.12
‘=6,6 +l”6,5
28353.66
0.03
116,5 +l”6,4
28378.12
0.02
%,3
t1O8,2
28113.12
0.00
ll9.2 +109,1
28048.06
0 .Ol
121,11 +111,10
27589.58
0.04
122,11 +112,10
27541.04
0.18
“6,6
31101.86
0.02
‘*6,7 +116,6
31036.18
0.02
ll17,5
30858.53
0.11
127,5 +117,4
30861.86
-0.32
‘*8,4 +%,3
30732.77
-0.12
129,3 +119,2
30647.75
-0.04
1210,2 clllo,l
30587.67
-0.15
1211,1 tllll,O
30543.60
-0.15
130,13 c~*o,12
27544.42
0.05
29612.20
-0.04
29584.12
-0.04
31624.12
-0 .oo
“7,6
%,12
c121,11
l40,14 +130,13 l50,15 +140,14
(MHz)
*.-a.
=4,7
+116,5
67
oxygen-18 content of the carboxyl group of s,alicyl aldehyde was increased to approximately 40 % by exchange with excess acidified 40 % oxygen-18-enriched water. Both exchange reactions occurred readily in the liquid phase at room temperature. The salicyl aldehyde was separated from the water by vacuum distillation. The microwave spectra were recorded on a Hewlett-Packard model 8460A microwave spectrometer, within the range 26.5-40.0 GHz. The spectrum of the normal species is shown in Fig. 1.
JONES AND CURL TABLE II OBSERVED
TRANSITIONS-O-D
Tram ition
SALICYL ALDEHYDE
(MHz)
Observed
Obs-Calc
lL2,9 c102,8
27793.04
-0.12
ll3,v +lO3,8
26986.78
0.02
_lO3,7
29741.17
-0.06
114,8 *104,7
28011.26
0.05
114,7 +104,6
29559.14
0.08
%,7
+105,6
28206.20
0.13
%,6
+lO5,5
28507.05
0.15
ll6,5 +106,4
23085.65
0.10
116,6 +l"6,5
28061.62
0.14
117,4 +lO7,3
27919.85
-0.17
118,3*108,2
27823.80
0.00
119,2cLo9,1
27759.80
0.30
121,11elll,10
27309.33
0.04
122,11+112,10
27261.73
0.08
122,1Ll +112,9
29677.50
0.30
123,10+113,9
29142.93
0.01
123,9cL13,8
32091.51
-0.10
124,V +ll4,8
30430.19
0.05
124,8+l14,7
32559.42
0.10
"5,8 *115,7
30830.20
0.01
125,7+"5,6
31434.18
0.08
=6,7 +116,6
30716.32
0.06
126,6+116,5
30780.64
-0.22
127,6+'l7,5
30540.60
0.07
127,5 +117,4
30543.92
-0.32
128 4 +11 893
30416.36
-0.15
129,3 +119,2
30332.22
-0.19
1210,2tlllO,l
30272.90
-0.27
13 0,13 t120,12
27264.45
0.03
131,12c121,11
29311.22
-0.15
13 2,12 c122,11
29288.40
0.03
14 0,14 *l30,13
29283.46
-0.02
%,8
OBSERVED
TUNSITIONS
Transition
112,9cL02,8 LL3,8 *L"3,7 'L4,8*104,7 LL5,7*LO5,6 LL5,6+L05,5 LL6,6'L06,5 'L6,5+l"6>4 117,5*LO7,4 L18,4 +"8,3
TABLE: III -CHOl* S~LICYL ALDEHYDE
(MHz)
Observed
Obs-Calc
27350.00
0.26
29004.30
0.10
27314.29
-0.04
27435.01
0.07
27645.38
0.17
27291.92
-0.14
27306.90
-0.20
27167.05
0.05
27084.56
-0.08
27028.82
-0.20
26837.78
0.16
26770.50
0.00
29222.98
0.31
28566.20
-0.09
29707.36
0.23
31532.40
0.14
29906.45
-0.07
29865.80
0.02
29709.10
0.12
29601.25
-0.31
29528.65
-0.25
26779.52
-0.32
119,3+LOv,z 1*1,11+LLl,lO 122,ll+Ll2,10 122,lO+L12,9 12 3,lO +113,9 124,9*L14,8 124,8*"4,7 126 6 +'L6 5 126;7+116:6 127 6+117 5 "8,4 +118,3 129>3 +LL9,2 13 0,13+L20,12 131,12 +L21,11 i32,llCL22,lO L32,12+L22,11 140,14+L30,13 L42,13+132,12 142,12 +132,11 150>15 +140t14 L51,14*141,13 152,14'142,13 160,16c150,15 L70,17+L60,16
l
180,18 170,17
28798.75
-0.02
31074.65
-0.21
28764.92
0.07
28162.90
0.04
30752.94
0.04
32949.98
-0.26
30746.U
0.04
32745.38
-0.36
32737.72
-0.04
32729.62
0.19
34713.00
0.17
36696.38
0.11
69
JONES AND CURL
70
ANALYSIS OF THE SPECTRUM
Molecular model calculations indicated that salicyl aldehyde was a prolate asymmetric rotor with rotational constants of approximately A = 3380, B = 1,525, C = 1051 MHz, and Ray’s asymmetry parameter K = -0.59. The model also indicated that the a component of the dipole moment was the largest. Since the rotational constants were quite small, the a-type transitions which fell into the region 26.5-40.0 GHz had quite high values of the rotational quantum number J. As a result of this, little aid was gained from Stark effects. As may be seen from Fig. 1 u-type R-branch bands were prominent in the spectrum, and little difficult’y was experienced in identifying individual transitions within t,hese bands. Table I shows a least square fit of 25 lines for the normal species of salicyl aldehyde. Tables II and III show least square fits for 32 lines of C6H4. CHO.OD and 37 lines of CsH4. CHl*O. OH, respectively. In all three cases, the fit to a rigid rotor model was good and there appears to be no significant centrifugal distortion effect’s present. This is in keeping wiOh observations on nitrobenzene (6). Only a-type, R-branch transitions were measured for all three isotopic forms of salicyl aldehyde. However, the molecule was sufficiently asymmetric to allow accurate values of all three rotat’ional constants to be determined. Table IV TABLE IV ROT~TION.&L
CONST.INTS OF S.~I,ICYLALDEHYDE E!
A
(MHz)
c
Model
3249
1525
1038
Normal
3215.97
1493.618
1020.110
-0-D
3184.44
1478.184
1009.742
-CHOl8
3212.01
1434.501
991.779
Standard deviation in observed
0.06
0.004
0.004
TABLE V MOLECULAR
MOMENTS
OF INERTIA OF S~LICYL ALDEHYDE(AMU
AZ)
f Normal
157.194
338.461
495.565
-0.091
-0-D
158.750
341.994
500.653
-0.091
-MO18
157.387
352.409
509.721
-0.075
(conversion
factor 5.05531 x 105mi+.m.u.
A21
MICROWAVE
SPECTRUM
OF SALICYL
TABLE COORDIXATES Experimental
ALDEHYDE:
71
VI CA) a
b
Deuterium atom
+1.a73
-1.260
018 atom
+2.657
+0.32&
Model Deuterim atw
+1.97
-1.29
P
+2.60
40.33
atmn
shows the rotationa constants for all three isotopic species. The error limit quoted represents the standard deviation obtained from the least-square fits of the observed transitions. As expected, the rotational constant A was determined with less certainty than the other two constants. MOLECULAR
STRUCTURE
The molecular moments of inertia calculated from the observed rotational constants are shown in Table V. For a planar molecule the inertial defect, I,-I,-Ib , should be zero, but usually vibrational averaging effects cause the inertial defect to become nonzero. As can be seen in Table V, the inertial defect of all three isotopic species of salicyl aldehyde is very small and negative. The fact that the inertial defect is negative shows that out-of-plane vibrational motions predominate, these motions probably being the aldehyde and phenol group torsions. The magnitude of the defect, approximately -0.085 amu AZ, fits in well with observations on fluorobenzene (A = +0.03) (7) and nitrobenzene (A = -0.48) (6). There can be no doubt that salicyl aldehyde is planar. With the data available it was possible to locate the phenolic proton and the carboxyl group oxygen by use of Kraitchman’s equations (8). The resulting calculated coordinates are shown in Table VI. There seems to be little ambiguity about the sign of the coordinates since the signs shown place the two atoms extremely close to the positions indicated by the initial molecular model. The molecular model used was based on an X-ray diffraction study of salicylic acid (9). Dimensions used for the aldehyde group were those observed in formaldehyde (10). It was found that if the C-O bond of the phenol group was tilted so that it made an angle of 6” with a line bisecting the ring angle at that point, a model was produced which not only reproduced the observed rotational constants well (Table V) but also placed the hydrogen and oxygen atoms extremely close to the coordinates calculated from Braitchman’s equations (Table VI). A diagram of the molecular geometry is given in Fig. 2. This angle of tilt is in keeping with the X-ray studies on the potassium salt of o-nitrophenol (11) in which a similar angle (6’) was observed. Thus it would seem that an extremely long O-H bond is not present. The obvious way to show this conclusively is to study a sample in
72
JONES
AND
CURL
FIG. 2. Molecular model which fairly closely reproduces the normal species rotational constants and the principal axis coordinates of the phenol hydrogen and aldehyde oxygen. The coordinates determined for these at’oms from Kraitchman’s equations are indicated by X.
which the phenolic oxygen has been enriched with oxygen-l% This possibility was considered closely but subsequently abandoned because of the lack of an economically feasible synthetic route. The quantity determined directly in this study is the nonbonded (or hydrogen bonded) distance between th,e phenolic proton and the carbonyl group proton, this value being 1.76 f 0.01 A. The error quoted here reflects only the estimated experimental errors; no attempt has been made to allow for such effects as zeropoint vibration. This distance is considerably less than the sum of the van der Waal’s radii of hydrogen and oxygen, and in the absence of some stabilisation extremely large steric repulsions would be expected. DISCUSSION
Thus we have a situation attributed to the moderately
in which significant structural changes weak hydrogen bond of Zaminoethanol
have been (4) while
MICROWAVE
SPECTRUM OF SALICYL ALDEHYDE
73
0.
P-AMINOETHANOL
-10
r
SALICYL
ALOEHYDE
FIG. 3. Energy versus H.*.B distance for systems which have strong hydrogen bonds. H is the hydrogen bonding hydrogen and B is the base (in t,he case of salicyl aldehyde 0). The two points marked correspond to salicyl aldehyde and 2-aminoethanol. The fact that 2-aminoethanol has an H. . . B distance which corresponds to a rapid change of energy with distance indicates that there is a strong force attracting the H to the base (N) in %-aminoethanol. This strong attractive force may account for the observed bond lengthening. In the case of salioyl aldehyde the rate of change of energy with H. . .B distance is small, little attractive force is felt, and the 0.. +H distance is normal. Note that the hydrogen bonding energy of salicyl aldehyde is considerably greater than that of 2-aminoethanol according to the figure.
there appears to be no unusual feature of the structure of the strongly hydrogen bonding salicyl aldehyde. We rationalize this situation by the following arguments. Presumably, as the hydrogen bonding hydrogen (in these cases a hydroxy group) is brought up from infinity to the base, the energy decreases to a minimum and then increases. The minimum point corresponds to the observed structure of intermolecular hydrogen bonding systems and for strongly hydroge? bonding systems of the 0-H. . .O type is an 0. . .O distance of about 2.5-2.6 A (2.64 A in propionic acid dimer) (12) or to an H **.O dist’ance of about 1.6 A. This behavior is illustrated in Fig. 3. In 2-aminoethanol the molecular framework prevents the hydroxy hydrogen from coming closer to the nitrogen atom. Thus the hydrogen bonding forces are primarily attractive and these attractive forces account for the bond lengthening of 2-aminoethanol. In the case of salicyl aldehyde the H +. a0 distance is near the equilibrium point of intermolecular systems and the attractive and repulsive forces are nearly balanced. ACKNOWLEDGMENTS Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. This work was also supported by Grant C-071 of The Robert A. Welch Foundation.
74
JONES
A.ND CURL
RECEIVED : September ZO,l971 REFERENCES 1. K. M. M~RSTOKE~AND H. MOELLENDAL,J. iVoZ. Struct. 6,205 (1970). 2. K. S. BUCKTONAND R. G. AZRAK, J. Chem. Phys. 62, 5652 (1970). 3. R. G. AZRAK AND E. B. WILSON, J. Chem. Phys. 52,5299 (1970). $. R. E. PENN .~NDR. F. CURL, J. Chem. Phys. 66,651 (1971). 5. D. HEINERT AND A. E. MBTELL, J. Amer. Chew Sot. 81,3933 (1959). 6. J. H. HOG, L. NYGAARD, AND G. 0. SORENSEN,1. Mol. Struct. 7, 111 (1971). 7. L. NYGAARD, I. BOJESON,T. PEDERSEN,BNDJ. R.~RTRUP-ANDERSEN, J. Mol. Struct. 2, 209 (1968). 8. J. KRAITCHMAN,Amer. J. Phys. 21,17 (1953). 9. W. COcHR.IN, Acta Crystdogr. 6, 260 (1953). 10. T. OKA, J. Phys. Sot. Jap. 16,2274 (1960). If. J. P. G. RICHARDS,2. Kristallagr. 116,468 (1961). 12. F. J. STRIETER,D. H. TEMPLETON,R. F. SCHEUERM~N,.\NDIt. L. SASS, Acta Crystallogr. 16, 1233 (1962).