J . Phys. Chem. 1987, 91, 3200-3203
3200
the coordination of one4 or two solvent molecules.30 Our results suggest that there is an energetically accessible transition state through which these compounds could pass without the need of solvent to form a penta- or hexacoordinate transition state. Thus it is quite possible that the loss of optical activity is due to an energetically accessible edge inversion pathway. This can also be extended to silicon systems. There is one reported3’ experimental example (1) of a lowenergy inversion process at Si with AE = -26 kcal/mol. The
i
0’
\o 3
to be planar32from experiment while the edge inversion process in PF, has AE = 68.4 kcal/mol at the SCF level (53.8 kcal/mol at the MP2 level)., It has also been ~ u g g e s t e dthat ~ ~ ,compounds ~~ with the central fragment of 3 may also have a low barrier or even be planar33 although the latter point is in dispute.34
Acknowledgment. The authors thank Dr. T. H. Dunning, Jr. for the Sn basis set. Registry No. CF,, 75-73-0; SiF,, 7783-61-1; GeF,, 7783-58-6; SnF,, 1783-62-2.
2 1 value of 26 kcal/mol for AE is not inconsistent with the value of AE found for SiF4 since the ligands bonded to silicon are so different. Indeed, the lower value of 1 is consistent with what is observed in ADPO (2) as compared to PF3. ADPO is known (31) Martin, J. C.; Stevenson, W. H., 111; Lee, D. Y. In Organosilicon and Bioorganosilicon Chemistry: Structure, Bonding, Reactivity and Synthetic Application; Sakurai, H., Ed.; Ellis Horwood: Chichester, U. K., 1985; Chapter 13, p 141.
(32) Culley, S. A,; Arduengo, A. J. 111J . Am. Chem. SOC.1984,106, 1164. (33) (a) Meyer, H.; Nagorsen, G. Angew. Chem., Int. Ed. Engl. 1979, 18, 5 5 1 . (b) Nagorsen, G.; Meyer, H. Zbid. 1980, 19, 1034. (34) (a) Dunitz, J. D. Angew Chem., Znt. Ed. Engl. 1980, 19, 1034. (b) Schomburg, D. Angew. Chem., Znr. Ed. Engl. 1983,22,65. (c) Bibber, J. W.; Barnes, C. L.; Helm, D. V. D.; Zuckerman, J. J. Angew. Chem. Suppl. 1983, 688. (35) In this paper the periodic group notation in parentheses is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transition elements comprise groups 3 through 12, and the p-block elements comprise groups 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e.g., I11 3 and 13.)
-
Quantitative Infrared Spectroscopic Investigations of Hydrogen-Bond Cooperativity H. Kleeberg,* D. Klein, and W. A. P. Luck Fachbereich Physikalische Chemie, Philipps- Universitat, 0-3550Marburg, FRG (Received: October 31, 1986)
Infrared spectra of solutions of 1,4-butanedioland aprotic solvents B in CC14 and of methanol in aprotic solvents demonstrate the presence of cooperative effects. The OH frequencies of OH-OH-B complexes of 1,Cbutanediol in CCll and of methanol dimers in B are compared with those of 0H.s.B interactions. In comparison to complexes with only one H bond (Le., OH-OH or OH-B) the mutual cooperative influence of the formation of the second H bond amounts to about 20%. For short H-bonded chains like 1,4-butanediol dimers in CC14 the cooperative effect seems to increase to about 123%. The similarities of the frequencies in 1,4-butanediol dimers and crystalline alcohols may be interpreted in terms of cooperativity.
Introduction In 1957 Frank and Wen’ postulated qualitatively the presence of cooperative effects between H bonds. This means that, for example, an existing H bond between an alcoholic O H group and an aprotic H bond acceptor B (Le., OH-B) would be strengthened by the formation of a second H bond between another alcohol R O H and the OH group already involved in ROH-B (Le., OH-OH.-B). On the other hand the strength of the H bond between the two alcohol molecules in O-H...O-H...B
/ R
/
R
would also depend on the H-bond acceptor strength of B. The presence of cooperative H-bond effects has been supported by theoretical calculations.*” Some theoretical papers concluded “the nonadditive component of the interaction energy to be small”.7 Therefore, experimental tests seem to be necessary. Some experimental indications were found for HF* and for alcohol^.^ ‘Presented in part at the Tenth International Conference on Non Aqueous Solutions, Leuven, Aug 18-22, 1986, and the Gordon Conference, New Hampshire, Aug 4-8, 1986.
0022-3654/87/2091-3200$01 S O / O
Matrix alcohol and water spectra lead to the conclusion that cooperative effects exist.1° Recently we found” that the usual OH frequency shifts Au~H-.B (AuOH...B = ugas - uOH ...B) of DOH-B complexes (B = H-bond (1) Frank, H. S.; and Wen, W.-Y. Discuss. Faraday SOC.1957.24, 133. (2) Kollman, P. A.; Allen, L. C. J . Am. Chem. SOC.1970, 92, 753. (3) Del Bene, J.; Pople, J. A. Chem. Phys. Lett. 1969, 4, 426. (4) Hankins, D.; Moskowitz, J. W.; Stillinger, F. H. Chem. Phys. Lett. 1970, 4, 527. (5) Clementi, E. Lecture Notes in Chemistry; Springer Verlag: Berlin, 1976; Vol. 2. (6) Schuster, P. Angew. Chem., Znt. Ed. Engl. 1981, 20, 546. (7) Clementi, E.; Kolos W.; Lie, G. C.; Ranghino, G. Int. J. Quantum Chem. 1980, 17, 371. (8) Couzi, M.; Le Calv&,J.; van Huong, P.; Lascombe, J. J. Mol. Slruct. 1970, 5, 363. (9) Symons, M. C. R.; Thomas V. K.; Fletcher, N. J.; Pay, N. G. J. Chem. SOC.Faraday Trans. 1 1981, 77, 1899. Symons, M. C. R. Acc. Chem. Res. 1981,14, 179. Symons, M. C. R., Fletcher, N. J.; Thompson, V. Chem. Phys. Lett. 1979, 60,323. (10) Luck, W. A. P.; Schrems, 0. Horizons in H-bond Research, VIth European Workshop, University of Leuven, Leuven, Aug 22-27, 1982. (11) Kleeberg, H.; Heinje, G.; Luck, W. A. P. J . Phys. Chem. 1986, 90, 4427. Heinje, G. Thesis, University of Marburg, Marburg, FRG, 1986.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 12, 1987 3201
Investigations of H-Bond Cooperativity TABLE I: Concentration Ratio
solvent no.
-
ce/cdl, of 1,4-Butanediol Solutions (cdbl 0.003 mol/L) in CC14 and Aprotic Solvents B" solvent B cB/cdiol VB~OH YB
cc14
1
nitromethane (NM) acetonitrile (ANL)
2 3 4 5 6 7 8 9 10
1258 467 390 113 71 35 21 42 55
acetic acid ethyl ester (AEE) acetone (AON) diethyl ether (DEE) tetrahydrofuran (THF) dimethyl sulfoxide (Me2SO)
pyridine (PYR) triethylamine (TEA)
3637 3600 3560 3558 3522 3498 3483 3425 3360 3205
-
3636 3594 3540 3537 3510 3468 3380 3340 3255 f 1 5 3140 50
vox
3476 3430 3417 3428 3426 3404 3470 3405 3350 f 15 3355 f 15
*
"The OH-B frequencies u B u 0 H (cm-I) of 1-butanol (cROH 0.01 mol/L) in CC14 + B ( I O S cB/cRoH5 60), uB and vox (cm-I) of 1.4-butanediol solutions are compared ( T = 25 "C). acceptor) are enlarged in the presence of cations to values Au+ proportional to A v O H ...B Au+ CAVOH ...B = (1 + ~ ) A V O...BH (1) with the following a values": K+ (0.08) < Na+ (0.40) < Li+ (0.55) 5 Ba2+(0.61) < Mg2+ (1.13). The cation interaction with H O D seems to polarize the O H group, thus strengthening the OH-B H bonds. In this paper we present experimental results for the analogous cooperative effect of intra- and intermolecular H bonds of alcoholic OH groups, which, to our knowledge, has not been demonstrated quantitatively by experiments.
-
Cooperativity of Intramolecular H Bonds of 1,4-Butanediol . In solution of l,4-butanediol in CC14 at low concentrations (c 0.003 mol/L) a large number of alcohol molecules form intramolecular H bondsI2J3 like
-
"ox
-
280
56L
E
+
i
\
N
E
9 8
2857
l.L-BUTANEDIOL
0
2 7
2703
2LO
3030
1 [nm
226
B IN CCI, T=25" C
200
14,
P 'i:
b
2
4 0 ,
160
120
"I
0 -H * * * O - H
\(CH1)4 /
This is known from the presence of two IR bands (vf 2 3636 cm-' and vox = 3476 cm-I), which are assigned to free and bonded OH, respectively (see Figure 1, upper spectrum). In addition diol molecules with two free O H groups, HO(CH2)40H may be present. If H-bond acceptors B are added to this solution (see Figure 1, full lines) complexes
80
LO
0 -Ha-. OH . e . B
\(CH2)4 /
0
are formed. This can be concluded from the appearance of two new bands in the IR spectrum. In order to separate the spectrum of the intermolecularly H-bonded complexes from that of isolated 1,Cbutanediol molecules, we subtracted a fraction a' of the spectrum of the binary solution (see Figure 1, upper curve) from the spectra of the ternary solutions (diol in CC14 B). The condition of the choice of a' was that the extinction coefficient tOHin the region of vf (3636 cm-I) becomes about zero. In these difference spectra (AZOH, dotted lines in Figure 1) two separate bands vox and vB are present. They are due to diol-B complexes 1. In some cases (see below)
+
1
one broad band is due to the overlapping of the two bands. As a measure of the H-bond acceptor strength of B we may use the OH frequency vBuOH of 1-butanol complexes (CH3(CH2)30H-B) in CC14. In dilute alcoholic solutions in organic solvents B the OH frequencies (OH-B) of 1,4-butanediol and 1-butanol correlate linearly with a slope of 1. With an appropriate ~
(12) Kuhn, 86, 650.
~~
L.P.J. Am.Chem.Soc. 1951, 74,2492; 1954,76,4323; 1960,
(13) Luck, W . A. P. Nafurwissenschaften1967, 54, 601.
300
3700
3500
3300
1100
v [em"
Figure 1. OH stretching band of 1,4-butanediol (0.003 mol of diol/L) in CCI4 without bases B (at the top) and in the presence of B in CCI, (full lines, eoH)at 25 OC. Dotted spectra: weighted difference spectra (AZO,) of 1,4-butanediol in CC&. Y Y , vox, and vB are indicated by *, t, and t, respectively. For abbreviations and concentrations see table.
choice of the concentration ratio cB/cdiOl (see Figure 1 and Table I) the formation of HO(CH& OH-B or B-.HO(CH,),OH-.B complexes is negligible. Thus we use the 1-butanol frequencies vBuOH as a reference (see Figure 2). In diol-B complexes vf becomes vg This vB is plotted in Figure 2 as function of the vBuoH with the same bases B. The slope of the resulting linear correlation is c = 1.2. That means a similar cooperativity relation like eq 1 with aB = 0.2. It is due to the influence of the intramolecular H bond of 1,4-butanediol on its intermolecular H bonds to B. If we plot the intramolecular O H frequency vox as a function of vBu0H of (1-butanol-B) as a measure of the acceptor strength of B we observe a shift of vox by the mutual influence of the OH-B H bond on the lone pair, too. We find vox 0 . 2 v B u O H (Le., aox = 0.2). Without the mutual cooperative influence we would of course expect aBand aox to be about zero. Thus we may conclude that the H-bond cooperativity for complexes of 1 in CCI4 is about 20%. The results of Figures 1 and 2 show that vB is more sensitive to the acceptor strength of B than the intramolecular H-bond vox,
-
Kleeberg et al.
3202 The Journal of Physical Chemistry, Vol. 91, No. 12, 1987
ho
SMeOH
1,L-Butanedtol - 1-Butanol i n CCIL+B, T=25'C
[cm-' 31N
Methanol
Icm-ll
slope
i n B; T=2S°C
slope:
330C
330c
3500
350C
I f 2
I 3;
/
,
?
t
1
1
2
3$
1
+ 1
t
5 6 7 3500
8
1
9
~
IC m-' ~1
8
7
9 3300 & $,c.m Il'
3500 MeOH B
10
33'00 9
3
I
3;
9
1-BuCH B
Figure 2. The positions of the two O H bands of 1,4-butanediol-B complexes (see Figure 1) are plotted as a function of the position of the O H frequencies of 1-butanol in the presence of the same B (all values in dilute CCI, solutions; different B are indicated by numbers; see Table I). The slopes are a measure of the cooperativity between the intra- and the intermolecular H bond of the diol.
which changes only by cooperativity effects. Therefore, in Figures 1 and 2 vB shifts from 3636 cm-I (CC14) to about 3140 cm-I (triethylamine), and crosses the position of vox, which shifts from 3476 cm-I (CC14) to about 3355 cm-' (triethylamine). In binary systems of
~
Figure 3. OH stretching frequencies vox and v B of (CH,)OH.-(CH,)OHa-B complexes in different solvents B (indicated by numbers; see Table 1 1' refers to 1,2-dichloroethane) are found in weighted difference spectra (eOH (0.02 5 XcH,,H 5 0.04) - ( Y ' c ~ ~ , (=X0.01)) ~ ~ ~at O different ~ mole fractions of methanol X,H,oH. Abscissa: OH-stretching frequencies of = 0.01. methanol-B complexes Y M ~ O Hat XCH,OH 2561
E
' N
-E 6 0 1
2703
2857
L [nm
3030
1 .L-BUTANEDIOL IN CCl,(o=0.0066
26
molll)
0
z 5 LO
T['C
a
1
O H * * * OH
\
(CH,),
/ 20
in CC14 the polarization of the free O H group by the intramolecular H bond changes the frequency uf linearly with n.I2-l4 This change depends on the strength of this intramolecular H bond, which increases from n = 2 til n = 4 by the H-bond distances and angles which are both optimal for n = 4.
Cooperativity of Intermolecular H Bonds of Methanol It is useful to compare systems with OH-B and OH.-OH--B complexes of the same alcohol. This is possible by investigating the concentration dependence of the methanol spectra in aprotic solvents B (see also ref 9). Such spectra and their differences at different concentrations were compared: at low methanol mole fractions (XMeOH LZ 0.01) MeOH-B complexes (vMeOH...B) are present. At slightly higher concentrations (0.02 5 XM,, 5 0.04) MeOH self-association starts. In the difference spectra two new bands are observed, which are assigned to vox and uB in OH - * * OH *
I CH,
I CH,
0 -
B
complexes. There are straight-line correlations of vox and vB with YM,H.-B in the same solvents (see Figure 3). These experiments give aB = 0.33 and nox = 0.1 1. This indicates that the cooperative (14) Luck, W. A. P. In The Hydrogen Bond in Water: A Comprehensive Treatise; Franks, F., Ed.; Plenum: New York, 1973; Chapter 4, Vol. 11, pp 235-321. Luck, W. A. P. In Structure of Water and Aqueous Solutions; Verlag Chemie/Physik: Weinheim, 1974; Kap. 11.3.
0
-
3900
3700 Y
[om-'
2 b
i
i
, ? 4 3700
3500
3300
3100
om-'^ Figure 4. (a) O H stretching mode of 1,4-butanediol (0.0066 mol/L) in CCll at different temperatures. At 4 5 OC the spectrum is due to diol monomers (compare Figure 1, upper spectrum). (b) Differences between spectra at 5 and 2 5 OC and the one at 45 OC. With decreasing temperature the absorbance of free O H decreases (t) and that of open ends (1) and H-bonded OH (3455, - 3 3 5 5 , and 3 2 7 0 cm-I) increases. v
influence of the OH-0 interaction on OH--B is larger than that of the formation of OH-B on OH-0 in this case.
J . Phys. Chem. 1987, 91, 3203-3207 Cooperativity of Intermolecular H Bonds of 1,4-Butanediol Figure 4a shows the temperature dependence of the spectra of 1,Cbutanediol in CCll at a concentration of 0.0066 mol/L. At 45 O C the bands at vf = 3636 cm-I and vox z 3479 cm-I are present. With decreasing temperature an additional absorbance with a maximum at about 3280 cm-' increases strongly. Intermolecular aggregates, Le., 1,Cbutanediol dimers like
3203
Conclusions The presence of cooperative effects in inter- or intramolecular H bonds can clearly be demonstrated by the analysis of infrared spectra. The mutual cooperative influence in OH-OH-B and OH.-OH.-OH.-OH complexes is about 20 and 123%, respectively. This is similar to the cooperativity found in cation-OH-B complexes (see above). These results help to interpret the matrix spectra of small clusters of different size. Cooperative effects may be of considerable importance in the understanding of the structure and dynamics of liquids and solutions containing H bonds. They may play a central role in the action of biomolecules.
iCH 't iCH2)'\
O-H...O-H.."O-H...O-H v , Cm-'
* 53445
-3270
53355
1.3626
may be formed in addition to the diol monomers. The given frequencies are based on the interpretation of differences between spectra taken a t different temperatures (Figure 4b).I5 In pure liquid 1,4-butanediol at 25 OC, we find vmsx = 3330 cm-I. From the similarity of this frequency and that of the 1,4-butanediol dimer we may conclude that the OH groups with vOH ~ 3 2 7 cm-' 0 (see Figure 4) have already "full" cooperativity; Le., in four linearly H-bonded OH groups the central OH has a similar frequency as in crystalline alcohols (3230 cm-I 16) where cooperativity is complete. This means the additional frequency shift (cm-I) of the 1,rl-butanediol dimer: vox(intramolecular) v,(diol dimer, intermolecular) = 3476 - 3270 = 206 is 1.23 times larger than the shift due to the formation of the intramolecular H bond: vf - vox (intramolecular) = 3643 - 3476 = 167. This indicates that the cooperativity in H-bonded chains may amount to about 123%. (15) Kleeberg, H.; Luck, W. A. P.; Nowak, M. J.; Rangsriwatananou, K., manuscript in preparation. (16) Tauer, K. J.; Lipscomb, W. N. Acta Crystallogr. 1952, 5, 606. JBnsson, P.-G. Acta Crystallogr. B 1976, 32, 232. Mikawa, Y . ;Brasch, J. W.; Jakobsen, R. J. Spectrochim. Acta, Part A 1971, 27A, 529.
Experimental Section 1,CButanediol (Aldrich) and solvents (Merck, Darmstadt) were at least of reagent-grade quality. They were used after careful drying over molecular sieves of appropriate pore diameters. The absence of H20was confirmed spectroscopically for all substances before use. Thermostated quartz-Infrasil (QI: Hellma, Miillheim) cells (25.0 f 0.2 "C) of appropriate path length were used. Spectra were recorded on a Perkin-Elmer 325 spectrometer and digitized directly. Pure solvents or solutions without alcohols were used for compensation in the reference beam. The extinction coefficient tOH was calculated by bH= E/cd (E, absorbance; c, OH concentration in mol of OH/cm3; d , optical path length in cm). Acknowledgment. We thank P. Bopp, Darmstadt, for reading the manuscript, F. Wagner and F. Hohmann for technical assistance, and the Deutsche Forschungsgemeinschaft, Bonn, for funding this project. Registry No. N M , 75-52-5; A N L , 75-05-8; AEE, 141-78-6; A O N , 67-64-1; DEE, 60-29-7; THF, 109-99-9; Me,SO, 67-68-5; PYR, 11086-1; TEA, 121-44-8; 1,4-butanediol,110-63-4.
Ab Initio Study of the Gaseous Oxyacids of Phosphorus, Their Conjugate Bases, and Their Corresponding Neutral Radicals Lawrence L. h h r * and Randall C. Boehm Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109 (Received: November 3, 1986)
Ab initio calculations have been carried out to determine molecular geometries and energies for a number of neutral and anionic species involved in the gas-phase chemistry of phosphorus and oxygen. These species include the acids HOPO, HPOZ, HOPO,, and HOP (triplet state), the anion PO3-, and several electronic states of the radical PO3, thus complementing our earlier theoretical studies of Poz,POT, Po, Po-, and HPO. Energies were calculated by third-order Mailer-Plesset perturbation theory with a split-valence basis set augmented by both polarization and diffuse functions. Geometries were obtained at the self-consistent field level by analytical gradient techniques using a similar basis set without diffuse functions. Vibrational frequencies were calculated from analytical second derivatives. Key results are the prediction of an extremely large electron affinity (5.4 f 0.2 eV) for PO3 and of strong acidity for HOP02, these results being consistent with the observed gas-phase thermodynamic and kinetic properties of PO3-. A discussion is given of the acid-base properties of the species HPO, ( n = 1, 3) and of the electron affinities of the corresponding radicals PO, (n = 1, 3).
I. Introduction The study of the combustion of phosphorus and phosphoruscontaining compounds has continued to be an important area of research.i-6 M~~~ of the reactions are accompanied by the (1) Fraser, M. E.: Stedman, D. H. J . Chem. Soc., Faraday Trans. 1 1983, 79, 521. (2) Fraser, M. E.; Stedman, D. H.; Dunn, T. M. J . Chem. SOC.,Faraday Trans. I 1984, 80, 285. (3) Henchman, M.; Vigianno, A. A.; Paulson, J. F.; Freedman, A.; Wormhoudt, J. J . Am. Chem. SOC.1985, 107, 1453. (4) Harris, D. G.; Chou, M. S.;Cool, T. A. J . Chem. Phys. 1985.82, 3502.
0022-3654/87/2091-3203$01.50/0
emission of visible light, with the list of species identified or suggested as giving rise to these emissions including PO, HPO, POz, HOPO, and (Po),. In a recent theoretical study7 we presented the results of our ab initio investigations of some of these species, with the emphasis on the characterization of the oxides PO2 and PO together with their anions. A key result was the prediction of a very large electron affinity (EA) for Po*, namely (5) Hamilton, P. A,; Murrells, T. P. J . Chem. Soc., Faraday Trans. 2 1985, 81, 1531.
(6) Hamilton, P. A.; Murrells, T. P.J . Phys. Chem. 1986, 90, 182. (7) Lohr, L. L. J . Phys. Chem. 1984, 88, 5569.
0 1 9 8 7 American Chemical Society
