650
W. V. F. BROOKS AND CLYDEM. HAAS
Intramolecular Hydrogen Bonding and Potential Functions of Carboxylic and Percarboxylic Acids
by W. V. F. Brooks and Clyde M. Haas
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Department of Chemistry, Ohio University, Athens, Ohw
(Received August 2.9, 1966)
To study the nature of the forces between the acidic hydrogen and the carbonyl oxygen atoms in organic acids, a normal coordinate analysis has been carried out for monomeric formic, acetic, trifluoroacetic, performic, and peracetic acids. The results indicate weak 0 . . . H forces in the carboxylic acids and considerably stronger 0 . .H forces in the percarboxylic acids. 9
Introduction The planar configuration of monomeric carboxylic acids, with the acidic hydrogen cis to the carbonyl oxygen,'-' must be due to some combination of r bonding between the carbon and oxygen atoms, steric effects, intramolecular hydrogen bonding, intramolecular dipole-dipole interactions, etc. Since the intramolecular forces, however they are classified, are part of the molecular potential function, the determination of force constants from vibrational frequencies is a method for investigating the nature and the magnitude of the forces responsible for this planar configuration. Formic, acetic, and trifluoroacetic acids were chosen as typical carboxylic acids for which adequate data are available, and as molecules small enough to be handled by the available computing equipment.* Performic and peracetic acids were also treated because the percarboxyl group is probably also planar and the extra oxygen permits an arrangement more favorable to hydrogen bonding. While vibrational analyses of some of these molecules have been done previously,' # l o,11 no provision was made for forces acting between the acid hydrogen and the carbonyl oxygen. This study includes 0 . . - H stretching and additional bending coordinates.
Data and Computation The models used for the carboxyl and percarboxyl groups are shown in Figures 1 and 2 and the parameter values are given in Table I. The models and values are based upon electron diffraction datalaJ microwave data, 206 and an assumed percarboxyl structure.8 It is The Journal of Phyeicd Chemistry
also assumed that the carboxyl group is identical in the three carboxylic acids and that the methyl and trifluoromethyl groups are tetrahedral. The percarboxylic acid problems were simplified by replacing the methyl group of peracetic acid with a single particle
Table I: Structural Parameters of Molecules Studied Bond description
c-c C-F C-H
c-0 c=o 0-H 0-H
0-4
(per acids)
Length, A
1.54 1.35 1.09 1.36 1.25 0.97 1.02 1.49
Angle description
R-C=O R-C-4 C-0-H
c--o-o 0-0-H
Angle, degrees
125 110
107 105 100
(1) T. Miyasawa and K. S. Pitzer, J. Chem. Phys., 30, 1076 (1959). (2) G. H. Kwei and R. F. Curl, Jr., ibid., 32, 1592 (1960). (3) I. L. Karle and J. Karle, ibid., 22, 43 (1954). (4) J. H. N. Loubser, ibid., 21, 2231 (1953). (5) J. Karle and L. 0. Brockway, J. Am. Che-m. Soc., 66, 574 (1944). (6) R. E. Kagarise, J. Che-m. Phys., 27, 519 (1956). (7) M. Davies and 0. Thomas, Discussions Faraday Soc., 9, 335
(1950).
(8) IBM 162CL-I computer with 40,000 digits of core memory. (9) P. A. Giguere and A. W. Olmos, Can. J. Chem.,30, 821 (1952). (IO) (a) 0. Thomas, Discussions Faraday Soc., 9, 339 (1950); (b) K. Nakamoto and S. Kishida, J. Chem. Phys., 41, 1554 (1964). (11) R. Blinc and D. Hadsi, Spectrochim. Acta, 12, 82 (1959).
INTRAMOLECULAR HYDROGEN BONDING
651
Table I1 : Force Constants" for Carboxylic and Percarboxylic Acids Coord no.
R-C
1
2 3 4 5 13
c=o
1
c-c C-0-H o=c-0 c-0-0
...
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Bonds
9 or 10 8 9 10or 11 11 or 12 I
.
.
... 6 7
... ... ... '1
2,3 ...
...
3, 13 I1
3,7 3,9 2,9
... ...
...
C-0 0-H 0 .. * H 0-0 C-H or C-F
or (0-0-H)
0-Ha '.O C=O.. . H or F-C-F H-C-H H -C- C or F-C-C R-C=O R-C=O R-C out-of-plane wag C-CXa torsion COOH out-of-plane bend R-C / C 4 c=o/c-Q C-F/C-F C-C/C-F
HCOOH
CHiCOOB
4.728 9.609 5.203 7.162 0.011
4.784 9.451 5.046 7.265 0,094
4.958 9.925 5.609 7,238 0,005
4.728
4,876 4.784 0.736 1.128
5.657 4.958 0.847 0,830
0.0 0.0 0.518 0.716 1.734
1.511 0.894 0.027 0.653
0.0 0.0 1.220 1.121 1.627 1.225 0.702 0.027 0.582
1.106
0.341
0.744 2.i~ 0.0 0.0
0.602 0.702 0.387 0.496
HCOOOH
4.819 9.004 5.453 5.910 0.660 4.284 4.819 0.488 2.302 0.729 0.108 0.961 0.612 0.622
CHaCOOOH
3.924 9.293 5.200 5.659 0,525 2.832 3.924 0.375 1 .545 0.762 0.191
I.307 1.747 1.657
1.005
-0.365 1.665 1.024
c-o/o-o
R-C/O=C-0 C--O/R-C--O C--O/Ca-H C==O/C--O-H R-C / H - C 4 H-C-C/H-C--C R-C wag/COOH outof-plane
Moleoules CFaCOOH
-0.753 0.328 +O .425 0.112 -0.132
-0.593 0.134
0.510 0.138 0.434 -0,009 0.117
0.223
" Units are mdyneslh for stretching constants] mdyne A/( radian)* for bending constants, and mdynes/radian for bend-stretch interaction constants.
Figure 1. Carboxylic acid coordinates.
Figure 2. Percarboxylic acid coordinates.
Volume 71, Number S February 1987
W. V. F. BROOXS AND CLYDEM. HAAS
652
Table I11 : Selected Force Constant Values” Value
Source
RCOOH RCOOOH RCOOH (RCOOH), HNOa HCOs- (cryst) HCOsCHaOH HzO HzOz
7.2 5.8 6.9* 4,7* 7.1 3.2* 6.5 7.6 8.1 7.3
This work This work b
RCOOH RCOOOH HOOCCH=CHCOa (RC0OH)t HCOa- (cryst)
0.06 0.6 1.1* 0.4* 0.76,
This work This work j
Bond
0-H
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0
e
Molecule
stretch
.H stretch
C=O stretch
-
RCOOH RCOOOH RCOOH (RC0OH)a H&O HCOD-
..*
C - 0 stretch
RCOOH RCOOOH RCOOH (RC0OH)z HCOaCHSOH
...
X-0-H
(X = or H )
bend
c, 0,
RCOOH RCOOOH HOOCCH=CHCOzHCOaHNOs CHsOH Hz0 HzOz
C
d e
f B
h
i
C
e
9.7 9.2 11.2, 10.0* 12.4 9.1 11 .8-13.4
This work This work b
5.3 5.3 4.6* 4.0* 5.45 f 0 . 1 5 5.15 5.0-5.8
This work This work b
0.8 0.44 0.1 U.84 0.88 0.79 0.70 0.90
This work This work j
C
k
f
1
C
f ff
k
f d
B h
i
’ Values are rounded and averaged. Units are the same aa in Table 11. Values followed by * are from Urey-Bradley force fields. G. E. RiIcGraw, D. L. Bernitt, and I. C. Hisatsune, J . Chem. Phys., 42, 237 (1965). e K. Nakamoto, Y. A. Ref lob. Ref 16. T. Oka and Y. Morino, J. Mol. Spectry., 8, 9 (1962). P. A. Ref 18. ’ Ref 14. Sarma, and H. Ogoshi, ibid., 43, 1177 (1965). Giguere and 0. Bain, Can. J . C h a . , 56,340 (1951). K. Nakamoto, Y. A. Sarma, and G. T. Behnke, J. Chem. Phys., 42, 1662 (1965). T. Oka and Y. Morino, J . Phys. Soe. Japan, 16, 1235 (1961). Ref 12, p 175. Ir
’
of mass 15.035 and treating only the in-plane vibrations. The bond coordinates for in-plane motions are shown in Figures 1 and 2. The only unusual feature is that the acid hydrogen atom is treated as equivalent to the other atoms in a four (carboxyl)- or five (percarboxyl)-membered ring system. All the bond coordinates are listed in Table I1 with the final values of the force constants. For the out-of-plane coordinate of the carboxyl group, a bending of the four-membered The Journal of Physical Chemistry
ring was chosen instead of the usual torsion coordinate in order to avoid making an early judgment as to the kind of motion involved. The Wilson F and G matrix methodI2 was used with a valency force field. G matrix formulation, solution of the secular equations, and determination of the best values of the force constants were done by digital (12) E. B. Wilson, Jr., J. C. Decius, and P. C. Cross, “Molecular Vibrations,” McGraw-Hill Book Co., Inc., New York, N. Y . , 1965.
INTRAMOLECULAR HYDROGEN BONDING
653
Table IV : Calculated and Observed‘ Frequencies of Formic Acid Assignment
-HCOOHObsd
0-H stretch C-H stretch C=O stretch H-C bend C-0-H bend C-0 stretch H- C out-of-plane COOH out-of-plane O=C-O bend a
3570 2943 1770 1387 1229 1105 1033 669 638
Calcd
3583 2958 1796 1383 1244 1125 1035 674 633
----HCOOD-Obsd
2632 2948 1772
... 990 1178
... 529 573
Calod
2613 2959 1776 1367 975 1172 1033 524 569
-DCOOHObsd
3570 2220 1756 970
... 1143
... 661 632
-DCOOD-Calod
Obsd
Calcd
3584 2224 1747 972 1240 1152 870 665 631
2632 2232 1742 1040 945 1171 873 515 569
2613 2223 1722 1034 934 1161 871 508 568
Observed frequencies from ref 15 except for two lowest frequencies of each molecule which are from T. Miyazawa and K. S. Pitzer,
Downloaded by UNIV OF GEORGIA on September 5, 2015 | http://pubs.acs.org Publication Date: February 1, 1967 | doi: 10.1021/j100862a027
J . Chem. Phys., 32, 1592 (1959).
Table V : Calculated and Observed’ Frequencies of Acetic Acid
Assignment
--
0-H stretch C-H stretch C-H stretch C=O stretch H-C-H bend H-C-C and H-C-H bends C-0-H bend C-0 stretch H-C-C bend CH3 rock C-C stretch 0-H rock CH3-C bend O=C-0 bend CH3 torsion
a
CHsCOOH--7 Obsd Calcd
---
CHsCOOD-Obsd Calod
CDsCOOH-Obsd Calcd
e -
7-CDaCOODObsd
3546 3027 2935 1770 1431 1403
3610 3028 2910 1777 1439 1406
2653 3021 2940 1770 1434 1403
2632 3032 2910 1770 1440 1403
3640 2225 2111 1760 1060 1065
3610 2256 2097 1767 1037 1056
2660 2237 2111 1760
1284 1192* 1068* 996 846* 650 564 536
1316 1219 1053 968 853 673 561 536 221
974 1284 1057b 950 837b 546 564 535
968 1273 1053 940 822 509 559 519 22 1
1335 1217 820 926 790
1327 1221 814 968 802 673 495 531 167
1000 1280 812 925 785 ...
... ...
...
... 1060
... ...
Calcd
2632 2257 2097 1760 1037 1058 964 1282 813 940 781 509 486 532 167
... Observed frequencies from W. Weltner, J . Am. Chem. SOC.,77,3941 (1955), except where indicated otherwise. ’ J. K. Wilmshurst ...
e . .
...
J . Chem. Phys., 25, 1171 (1956).
computer, using modifications of Schachtschneider’s programs. l 3 The main program is designed to adjust the values of the force constants until calculated and observed frequencies are in the best agreement possible. The adjustment of force constants was done independently for each molecule so that, for example, each value for the 0-H stretching constant represents an independent determination. The only constant transferred from another molecule was the torsion constant for the methyl and trifluoromethyl groups which was taken from methan01.l~ The percarboxylic acids posed a problem since the data are insufficient to determine even the diagonal force constants uniquely. The calculations for performic acid illustrate the method used. Eight freFORTRAN
quencies are available and there are thirteen diagonal force constants to be found. A set of starting values of the thirteen constants was chosen, and ten of these were held fixed while the best values of the three “variable” constants were found. Then these three were fixed for the next step, and three other constants became “variables” and their best values were found. This was continued with different combinations of constants held fixed until calculated and observed frequencies agreed. This procedure cannot lead to reliable values of all the constants, and the values found (13) J. H. Schachtschneider, Technical Report No. 263-62, Shell Development Corp., Emeryville, Calif., 1962. (14) C. M. Haas, Thesis, Ohio University, 1965.
Volume 71, Number S February 1967
654
W. V. F. BROOKS AND CLYDE ill. HAAS
will in general be very much dependent upon the starting values chosen. Nevertheless, the results can give Significant values for constants (such as 0-H or C-H stretchings) of groups with normal vibrations that are separable or almost separable from the other vibrations of the molecule.
Table VI11 : Calculated and Observed" Frequencies for Peracetic Acid
Results and Discussion Table I1 lists the final values of the force constants and Table 111 gives selected force constants from this and other studies. The observed and calculated frequencies and assignments are listed in Tables IV through VIII. Downloaded by UNIV OF GEORGIA on September 5, 2015 | http://pubs.acs.org Publication Date: February 1, 1967 | doi: 10.1021/j100862a027
C HSCOOO H-----Calcd
I - -
a
Assignment
Obsd
0-H stretch C=O stretch 0 4 - H bend C-0 stretch 0-0 stretch C-C stretch O=C-0 bend CHa-C bend Hydrogen bond stretch
3310 1760 1450 1248 868 86 1 656
3310 1760 1450 1249 868 86 1 656 513 411
...
Reference 9.
Table VI : Calculated and Observed" Frequencies of Trifluoroacetic Acid
Assignment
0-H stretch C=O stretch C-0 stretch C-0-H bend C-F stretch C-F stretch C-C stretch CF3 rock 0-H rock O=C-0 bend CF3-C bend F-C-F bend F-C-F and F-C-C bends F-C-C bend CF3 torsion
---CFsCOOH-Obsd
3587 1826 141jb 1300 1244 1182 825 781b 661b 580b 507' 436'
Calcd
-CFaCOODObsd
2648 1823
...
3603 1836 1417 1307 1248 1170 821 790 654 585 500 440 326
... ...
271 53
...
. .
,
1039 1243 1187 798 781'
... ... ...
... I
.
.
...
Calcd
2626 1826 1412 1026 1249 1169 798 778 490 558 497 438 324 267 53
a Observed frequencies from N. Fuson, M. Josien, E. A. Jones, and J. It. Lawson, J. Chem. Phys., 20, 1627 (1952), unless indicated otherwise. * R. E. Kagarise, ibid., 27, 519 (1957).
Table VI1 : Calculated and Observed" Frequencies for Performic Acid
a
Assignment
Obsd
Calcd
0-H stretch C-H stretch C=O stretch 0-0-H bend H-C bend C-0 stretch 0-0 stretch O=C-0 bend Hydrogen bond stretch
3367 2987 1739 1453 1340 1243 a59 810
3367 2987 1739 1453 1340 1243 a59 810 437
Reference 9.
The Journal of Physical Chemistry
-
-
HCOOOH--
7 -
The agreement between calculated and observed frequencies is satisfactory except for the methyl group rocking and the out-of-plane motion of the carboxyl group in acetic acid. The perfect agreement for the percarboxylic acids is the result of having more force constants to adjust than the number of frequencies. The assignments for formic acid are in general agreement with previous assignments by Miyazawa and Pitzer,' Millikan and Pitzer,15 and Kakamoto and Kishida. lob The acetic acid assignments differ from Nakamoto and Kishida only in a reversal of the assignments for the 1284- and 1192-cm-' bands. The three sets of carboxyl group constants show some significant differences, especially in bending and interaction constants, but the 0 . . .H, C=O, C-0, and 0-H stretching constants are sufficiently consistent in value that it is appropriate to consider the three molecules together. The 0. .H stretching constant is the most obvious measure of hydrogen-bonding forces, and the low values (compared to formic and acetic acid dimers)16 indicate either weak forces or a situation in which opposing forces leave only a small re- 0bending sultant. The small C=O. . H and 0-H. constants seem to confirm this. The 0-H stretching constants have values slightly lower than would be expected if there were no hydrogen bonding. l7 The C=O and C-0 constants are of interest as indicators of the carbon-oxygen bonding. The C=O constants are roughly midway in value between singleand double-bond constants. This can be easily ex-
-
(15) R. C. Millikan and K. S. Pitzer, J . C h a . Phys., 27, 1305 (1957). (16) K. Nakamoto and S. Kishida, ibid., 41, 1558 (1964). (17) G. Herzberg, "Infrared and Raman Spectra of Polyatomio Molecules," D. Van Nostrand Co., Inc., New York, N. Y.,1951, p 193.
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INTRAMOLECULAR HYDROGEN BONDING
655
plained in terms of a delocalization of P electrons so that the C=O bond has only partial double-bond character. A shift of P electrons from one bond into another will presumably decrease the first stretching constant and increase the second, compared to isolated double and single bonds, respectively. However, a simple comparison of carboxyl group force constants with single-bond constants from alcohols and ethers may not be appropriate since both u and P bondings change in going from tetrahedral to planar configuration. Therefore, it is interesting that the C-0 force constants all have values within the range expected for carbonoxygen single bonds. The bicarbonate ion (HC03-) should have bonding similar to the carboxyl group but with P electrons delocalized among three carbonoxygen bonds. The reported C-OH stretching constantls for the bicarbonate ion is close to the average of the C-0 constants from the three carboxylic acids. The inertia defect19 was calculated for the ground state of formic acid (HCOOH); the calculated and observedz0 values are 0.07418 and 0.07709 amu/A*, respectively. Despite the uncertainties, the 0 . .H, O-H, C-0, and C=O stretching constants for the two percarboxylic acids are consistent and appropriate for molecules with strong intramolecular hydrogen bonding. The carbon-oxygen constants are also in agreement with the values from the carboxylic acids. The inconsistencies of some of the other constants serve as a reminder
-
that the data for the percarboxylic acids are not sufficient to define the potential functions.
Summary A valency force field, including constants for hydrogen bonding coordinates, has been found for each of the five acids studied. The values of the force constants indicate strong intramolecular hydrogen bonding in monomeric percarboxylic acids, weaker hydrogen bonding in the monomeric carboxylic acids, and degroups. localization of s electrons in the O=C-0 The partial double-bond character of the C-OH bond accounts for the planarity of the carboxyl group, but the bond does not show a significant increase in force constant compared to carbon-oxygen single bonds in other molecules.
Acknowledgment. The authors thank the Ohio University Research Committee for financial support, the staff of the Numerical Computation Laboratory a t the Ohio State University for their generous assistance, Dr. Jerry Schachtschneider for his computer programs, and especially the National Science Foundation whose grant supporting the Ohio University Computer Laboratory made this work possible. (18) D.L. Bernitt, K. 0. Hartman, and I. C. Hisatsune, J . Chem. Phys., 42, 3553 (1965).
(19) T.Oka and Y. Morino, J . Mol. Spectry., 6,472 (1961). (20) R. Wertheimer, Arch. Sci. (Geneva), 10, 184 (1957).
Volume 71,Number 3 February 1967
