3523
J . Phys. Chem. 1991,95, 3523-3527
Microwave Spectrum, Structure, and Dipole Moment of the Phosphorus Trifiuoride-Water Complex Marabeth S. LaBarge, Anne M. Andrews, Amine Taleb-Bendiab, Kurt W. Hillig 11, Robert L. Kuczkowski,* Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109- 1055 and Robert K. Bohn Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269-3060 (Received: September I I, 1990; In Final Form: November 20, 1990)
The dimer between phosphorus trifluoride and water was prepared in a supersonic molecular beam and observed with a Fourier transform microwave spectrometer. The rotational transitions for the PF3.H20and PF3.D20 species were split into doublets of unequal intensity, indicative of a tunneling motion which exchanges the H(D) nuclei. Only one tunneling component was observed for the PF3.HD0 species. Each tunneling component was fit to a semirigid Hamiltonian, including four centrifugal distortion constants. The dipole moment of PF3.H20was determined as pa = 2.308 D, p c = 0.283 D, and p(total) = 2.325 D. The moments of inertia and dipole moment components indicate that the dimer has C, symmetry with the hydrogen atoms and a pair of fluorine atoms straddling the symmetry plane. The water molecule is located over a PF2 face with the OH and PF bonds aligned in a pseudoeclipsed configuration.
Introduction The complexes of PF3with Ne, Ar, and Kr have all been studied recently by microwave spectroscopy. The PF3-Ar and PF3.Kr complexes have C, symmetry with the rare-gas atom located over a PF2 face,' and PF3.Ne has C3, symmetry with the Ne on the C3axis below the F3 face2 In the course of studying these species, the PF3-Hz0complex was unexpectedly observed. It arose as PF, apparently scavenged trace amounts of water adsorbed on the walls of the storage vessel and the inlet manifold to the spectrometer. (It is well-known that PF3 is relatively inert to H20.3) The structure of the PF3.H20 complex poses an interesting question. Complexes between PH3 and P(CH3)3with the acids HX4 and HCN5 are symmetric tops with the acidic proton directed toward the phosphorus lone pair. However, the hydrogens in H 2 0 are much less acidic, and the fluorines introduce a considerable electronic perturbation to the phosphorus as well as compete as a H-bonding site, making it difficult to anticipate the geometry of the complex between PF3 and H 2 0 . The microwave spectral data in this paper will show that the water molecule does not interact with PF3 via a linear H bond to either the phosphorus lone pair or a fluorine atom. The structure is similar to PF3.Ar with the oxygen replacing the rare-gas atom over a PF2 face. The O H bonds assume a position which pseudoeclipses the PF bonds.
Experimental Section Materials. PF3was obtained from PCR Chemical Co. and used without purification. Mass spectral analysis of the materials from the gas lecture bottle indicated no significant impurities within the sensitivity of the analysis (10.2%). The D 2 0 used in various experiments was stock material from Aldrich Chemical Co. (>99% D). Spectrometer. The rotational spectrum between 7.5 and 17.5 GHz was observed in a Fourier transform microwave spectrometer by using a modified Bosch fuel injector as a pulsed supersonic n o z ~ l e . ~Mixtures .~ of either PF3/H20/Ar or PF3/H20/Ne in ~~
( I ) Taleb-Bendiab, A.; LaBarge, M. S.;Lohr, L. L.; Taylor, R. C.; Hillig 11, K. W.; Kuczkowski, R. L; Bohn, R. K. J . Chem. Phys. 1989, 90,6949. ( 2 ) Hillig 11, K. W.;LaBarge, M. S.;TalcbBendiab, A.; Kuczkowski, R. L. Chem. Phys. Len. 1990, 171, 542. (3) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 2nd ed.; Interscience: New York, 1966; p 494. (4) (a) Legon, A. C.; Willoughby, L. C. Chem. Phys. 1983, 74, 127; (b) J. Chem. Soc., Chem. Commun. 1982,997; (c) J. Phys. Chem. 1983,87,2085. ( 5 ) (a) Legon, A. C.; Willoughby, L. C. Chem. Phys. 1984,85,443. (b) Hirani. H. L.; Legon, A. C.; Millcn, D. J.; Willoughby, L. C. J . Mol. Srrucr. 1984, 125, 171.
ratios of about 2:3:95 at total pressures of 1-2 atm were used to generate the spectrum. Except for some low-J transitions where unresolved spin-hyperfine splittings may have been present, the transitions had full widths at half-maximum of about 25-35 kHz. Center frequencies were typically reproducible to f 2 kHz and line center accuracies were estimated to be f4 kHz. Stark splittings were measured by applying dc voltages of up to f10 kV to two parallel steel mesh plates separated by about 30 cm. The 111-202 transition of SOz at 12256 MHz was used as an electric field calibration standard.* The Stark shifts of SOz and the complex were measured sequentially at each field value. Microwave Spectrum. During studies of the PF3.Ar system'*7 six low-J transitions split into doublets were observed that could not be assigned to that species. Since they appeared with either Ar or Ne carrier gases, they were suspected to arise from the PF3 dimer, with the spectrum complicated by internal rotation splittings. However, plausible structures could not be found for that dimer which were consistent with the J values indicated by the Stark splittings (J = 1 2 and 2 3). Upon returning to these six mystery lines after a considerable interlude, an assignment was found for them and additional transitions were predicted and observed. The derived rotational constants were close to those expected for an asymmetric PF3.Ne species, indicating that PF3 was complexed to a light moiety similar to Ne. Water was an obvious candidate, and when several drops of H 2 0 were added to the bulb containing the PF3-rare gas mixture, the transitions were considerably enhanced. The assignment of the PF3-D20and PF3.HD0 species along with dipole moment measurements and structure calculations made it quite certain that the PF3.H20 complex was the origin of the observed transitions. The PF3-H20spectrum was characterized by a dipole transitions split into doublets of unequal intensity separated by -0.5-30 MHz. Both components of each doublet had the same Stark splitting. The strong and weak sets of transitions could each be fit independently to a Watson S reduced Hamiltonian with Au,(obs-calc) I 8 kHz. The intensity ratio was approximately 3:l for the two components, suggesting a tunneling motion that exchanges two equivalent protons such as a rotation about the local C, axis of the H20. The transitions assigned to the strong and weak series of lines are listed in Table I as the A2 and A, states, respectively. The more intense component is usually found
- -
~
~
~~
~~
(6) Balle, T. J.; Flygare, W. H. Rev. Sei. Insrrum. 1981. 71, 829. (7) Hillig 11, K. W.; Matos, J.; Scioly, A.; Kuczkowski, R. L. Chem. Phys. Lett. 1987, 133, 359. (8) Patel, D.; Margolese. D.; Dyke, T. R. J . Chem. Phys. 1979,70,2740.
0022-3654191 /2095-3523$O2.50/0 0 1991 American Chemical Society
3524 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991
LaBarge et al.
TABLE I: Rotational Transitions for PF3.H20and PF3.D20 PF,*H20 J ’ K ; K ~ , - J ’ ’ K ~ ~ ~ K ~u,E~ ~MHz 202- 101 8 437.134 8 150.244 212-1 I 1 211-1 10 8 75 1.409 303-202 12 621 .I 60 313-212 12216.753 312-211 13118.040 322-221 12 675.501 321-220 12 730.535 404-303 I6 764.854 14-3 I 3 16 273.468 413-3 I 2 17473.229 423-322 16889.563 422-32~ 17025.884 432-311 16 925.535 431-330 16 927.904
PF,*D20 A2,‘ 3b
A,,. I b
Au,d kHz 0
MHz 8 431.961 8 140.092 8 755.71 1 12 607.999 12 200.205 13 123.052 12671.317 12735.049 16 737.760 16 249.129 17 477.044 16882.271 17039.739 16 924.363 16927.476 Y,
0
2 -1
2 0
-2 -1 0 0 1 0 0 4 -3
AI,O
Au, kHz -7 2 -3 -5 -5 -5 13 12 12 -8 -6 3 3 -3 -6
2b
v, MHz
Au, kHz
7845.415 7 589.830 8 122.070 1 1 741.378 1 1378.059 12 176.091 1 1 783.175 1 1 825.722 15 605.909 15 158.616 16221.309 15 702.262 1 5 807.873 15 729.770 15731.372
-1
-6 -5 0
0 -2 6 9 2 -2 0
0 2 -3 -1
A2,. I b MHz Au, kHz 7 846.222 -2 7 586.329 -1 8 129.546 -1 11 739.717 1 11 372.101 -1 12 186.547 -2 11 786.194 2 11 833.364 4 15598.531 2 15 149.448 -1 16233.771 -1 15705.401 1 15 822.344 0 15 736.076 -3 1 I5 738.009 u,
OState. bSpin weight. cMeasurement uncertainty is estimated to be 4 kHz. dObserved minus calculated frequency by using constants in Table 111.
TABLE 11: Rotational Transitions for PF3.HD0 u, MHz Au,’ kHz 8 137.705 - 1 1 7867.094 2 -1 8432.041 12176.188 -6 11793.056 -6 12640.089 -7 12222.929 14 15 12271.255
u, MHz A Y , kHz ~ 16179.085 14 15710.380 -8 16838.123 -6 16287.497 4 16407.497 3 16317.265 -6 16319.209 -5
‘Observed minus calculated frequency by using constants in Table 111.
at lower frequency, but not always. These symmetry labels are assigned by analogy to the H20-DOH dimer? where internal rotation of the acceptor H 2 0 occurs; we ignore the possibility of any internal rotation of the PF3 since no evidence was found for this motion. The rotation and distortion constants are given in Table 111. The assignment of the PF3.D20species is also given in Table I, and its spectral constants are in Table 111. Each of these transitions was also split into a strong and weak component. In this case the more intense set is labeled AI since Bose-Einstein statistics apply for an interchange of equivalent deuterons giving spin weights of 2:l. It is noteworthy that the relative positions of the A, and Az components is reversed for five transitions compared to the HzO species and that the tunneling splittings do not all decrease uniformally upon deuteration unlike other cases, such as H20*S0z’0and H2S.CO2,I1where a systematic decrease in splittings is observed upon deuteration. The situation is more analogous to H20O3I2(although not as severe) where the A,-A2 splittings were only slightly reduced (10-15%) upon deuteration. A Hamiltonian that included terms for a simple internal rotation of H20 (DzO) about its local C2axis could replicate the irregular splittings qualitatively, but the fits were quantitatively unsatisfactory and the AI component was always predicted at higher frequency. It contained a V2(1 - cos 2a)/2 term and kinetic energy terms analogous to those occurring in the internal rotation of methyl groups.13a The Av,,,,, (where Av = vObs- v - 3 was 4.6 (H20, V, = 125 cm-’) and 4.0 MHz (D20, V2= 169 cm-I). The (9) Coudert, L. H.; Hougen, J. T. J . Mol. Spectrosc. 1988, 10, 86. (IO) Matsumura, K.;Lovas, F. J.; Suenram, R. J . Chem. Phys. 1989, 91, 5887.
( I I ) Rice, J. K.; Coudert, L. H.; Matsumura, K.;Suenram, R. D.; Lovas, F.J.; Stahl, W.;Pauley. D. J.; Kukolich. S.G. J . Chem. Phys. 1990.92, 6408.
(12) G,illies, J. 2.; Gillies, C. W.; Suenram, R. D.; Lovas, F. J.; Cremer, D.; Schmidt, D. J . Mol. Specrrosc., in press. (13) (a) Swalen, J. D.; Herschbach, D. R. J . Chem. Phys. 1957. 27, 100. (b) Quade, C. R. J . Chem. Phys. 1967,47, 1073.
Avm8 decreased to 2.3 (59 cm-l) and 1.1 MHz (60 cm-I) when a distortion term D,,,,J(J l)m2 was included. Moreover, without exception, the correct ordering of the A I - A2 components was obtained and the distortion constants were more comparable to the values in Table 111. This suggests to us that internal rotation of the water is indeed the origin of the tunneling splittings but that the theory of Q ~ a d e , which I ~ ~ includes the dependence of the rotational constants upon the internal rotation angle (a)in the Hamiltonian, should be applied to determine the barrier. It is also possible that the barrier is too low for the Quade perturbation treatment to prove useful, and it may even be necessary to consider a more complex tunneling coordinate than a simple one-dimensional rotation about the local C2axis of HzO. In view of the formidable character of the Quade analyses and the possible complications, this matter was not pursued further. In addition, it would be helpful to assign some c dipole transitions and ascertain that pc does not change direction, as expected for a simple internal rotation of the H20. The small value of pc (see dipole moment section) has precluded success at this undertaking. The transitions from PF3.HD0 fall approximately midway between the PF3.Hz0and PF3.D20sets. They are listed in Table 11, and derived spectral constants are in Table 111. Only one set of transitions could be identified for this species. These transitions were weak and difficult to find since partial deuteration (using 50/50mixtures of H 2 0 and D20) usually gave weak PF3.HD0 and PF3.D20lines due to exchange losses in the inlet manifold. It is uncertain whether a second set of PF3.HD0 transitions are in the region searched but have been missed or whether they are severely perturbed and shifted to another region. Searches were made on three occasions for several target transitions in regions between the analogous transitions for the H 2 0 and D20 species; however, no unassigned lines have been found. Nevertheless, the possibility of missing a second state cannot be completely excluded. The transitions could, for example, be absent due to intensity limitations, for example if the second HDO species is an isomer with a different zero-point energy. This question will be further discussed in the section describing the structure analysis. Dipole Moment. The measured Stark coefficients for nine M components of three transitions for the A2 state of PF3eH20 are listed in Table IV. The Stark shifts were second order (within &2%). A least-squares fit to all three dipole components gave a negative value for pb2, which usually indicates that p b is small or zero. Setting p b 2 = 0 in the fit gave the dipole components listed in Table IV. Less accurate measurements for the AI state that exhibited similar second-order Stark shifts were also made; they indicated that this state has essentially the same values of pa2 and pc2. Structure of the Complex. The dipole moment components derived from the Stark effect measurements indicate that paand M~ are nonzero and suggest that the complex has an ac symmetry
+
The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 3525
Phosphorus Trifluoride-Water Complex TABLE III: Molecular Constants' for the Isotopic Species of PF3-Hz0
PFI.HZ0 parameter A, MHz B, MHz C, MHz Dj, kHz DJK,kHz d,, kHz dz, kHz
A, state 7014.871 (75)b 2263.062 (1) 1962.465 ( I ) 3.343 (1 I ) 30.500 (35) -0.496 ( 1 4) -0.143 (7)
PFq*DZO
A2 state 6544.780 (297) 2265.937 (4) 1958.108 (4) 3.271 (53) 20.865 (174) -0.568 (71) -0.237 (34)
A, state
A2 state
PF3.HDO
6938.850 (246) 2097.1 14 (2) 1830.979 (2) 3.052 (27) 33.041 (88) -0.487 (35) -0.136 (17)
6635.906 (92) 2100.351 (1) 1828.727 ( I ) 2.948 ( 1 2) 30.732 (38) -0.475 (15) -0.157 (8)
6964.271 (498) 2178.735 (5) 1896.241 (5) 2.953 (62) 72.395 (203) -0.568 (83) -0.461 (40)
Watson S reduction; representation P . buncertainties given in parentheses are 1 standard deviation and refer to last digit@)shown. TABLE IV. Stark Coefficients and Dipole Moments for the A State of PFq.H.0
transition 3(0,3)-2(0,2) 3( 1.3)-2( 1,2) 3(1,2)-2(1,1)
IM 0 1 2 0 1 2 0 1 2 1PaI IPcl
IPtmtl
Avlr2" -0.0833 -0.0225 0. I606 -0.0250 0.3848 1.5810 -0.0256 -0.3563 -1.3270 2.308 (2)c 0.283 (50) 2.325 (7)
obsd calcdb -0.0009 -0.0006 0.0008 -0.0007 0.0077 0.0006 -0.0007 -0.0048 0.0043
"Observed Stark coefficients in units of IO4 MHz (V/cm)-2. bobserved -calculated Stark coefficients. The latter obtained with the dipole components below. CTheuncertainties are lu. plane. This eliminates highly symmetric structures that align the Czand C, axes of the HzO and PF3 since they would have only a pacomponent. This configuration would also likely lead to free internal rotation from a presumably small V6 barrier and result in some transitions with first-order-like Stark effects, for which there was no evidence. If the complex has a symmetry plane, the oxygen, phosphorus, and one fluorine atom must lie in this plane. Two fluorine atoms will straddle the plane while the two hydrogen atoms may either lie in the plane or straddle it. Some insight on the two possible orientations for the HzO might be obtained by comparing the respective planar second moments ( P b b = CmibF) for the two orientations calculated from the known structures of PF3 and HZOl4with the observed values of P b b for the isotopes of PF3.Hz0. This is summarized in Table V. It is apparent that the experimental value for PF3-H20falls between the value of free PF3 (Le., both H's in a symmetry plane) and the calculated value for PF, H20 (Le., both H's straddle the symmetry plane), shedding little light on the location of the H atoms at first glance. This puzzling result arises from the large amplitude vibrations in this weakly bound complex, which introduce sizeable vibrational contributions to the moments of inertia. More insight is obtained by noting the changes in Pbb upon single and double deuteration, which are listed under A, and A2 in Table V. It is seen that the large changes upon successive deuteration are similar to the changes occuring in the S 0 2 * H z 0complex where the H atoms straddle a symmetry plane and are in contrast to the small changes in the 03.Hz0 complex where they lie in a symmetry plane. We conclude from these comparisons that the H atoms straddle a symmetry plane in the PF3.Hz0 complex. With this symmetry deduced, one puzzling observation deserves comment. This configuration should lead to tunneling doublets as the H and D atoms are interchanged in the PF3-HD0 species, just as in the HzOand D20complexes, except that the two components should have nominally equal intensities due to the lifting of nuclear spin restrictions. This second set of lines has not been found near where they are expected by comparison with
+
(14) Harmony, M.D.;Laurie, V. W.;Kuczkowski, R. L.; Schwendman, R. H.;Ramsey, D. A,; Lovas, F. J.; Lafferty, W.J.; Maki, A. 0.J . Phys. Chem., Ref. Data 1979, 8, 619.
TABLE V: Planar Second Moments, Pm" for Various Species (M) Complexed to H20,HDO,and D20in amu A' pbb
AIb
M M*H2O PF3(pred)c 52.651 0.0 0.588 PF3(pred)d 53.829 0.437 P F , ( ~ X ~ ) 53.125 ~ 03(exp)l
37.977
0.152
pbb
pbb
M-HDO
M*DZO
52.651 54.417 53.562 38.129 50.143
0.0 1.176 0.806 0.162 1.129
52.651 55.005 53.931 38.139 50.917
S02(exp)g 49.788 0.355 'Pbb = Em,!$,where b, is zero for atoms that lie in the uc symmetry plane. 'Ai = Pbb(M*HDO)- Pbb(M'H20). A2 = Pbb(M'D20) Pbb(M'H20). CPredictedfor the complex where the H20 (HDO, D20) atoms lie in a symmetry plane. PF, and H20 structures from ref 14 (ignores vibrational effects). dPredicted when the H (D) atoms straddle the symmetry plane and only oxygen lies in the plane. 'Experimental values, this work. 'Reference 12. The H20atoms lie in an ac symmetry plane. BReference IO. The H atoms straddle an uc symmetry plane. the H20 and D 2 0 species. There are three possible explanations. First, this may be due to an experimental oversight (present but missed). Second, a perturbation could shift the second component to a considerably different region of the spectrum. Indeed, additional terms are introduced into the internal rotation Hamiltonian for an asymmetric internal rotor such as HDO, which could have a pronounced effect on spectral patterns. In a related system, the HD0.S02 spectrum exhibited irregularities for this isotopic species,1° especially for the second tunneling state, and internal rotation splittings increased by factors of 100-200 over the H 2 0 and D 2 0 species. Third, in the PF3.HD0 species, there are no nuclear spin restrictions affecting the population of tunneling levels, unlike PF3.H20 and PF3.Dz0, where the two tunneling states have different spin symmetries. Consequently, the higher tunneling state may be greatly depopulated due to collisional relaxation at the low effective temperature achieved in a nozzle expansion (typically 2-5 K). We estimate that an energy difference of only 2-3 cm-' is needed to depopulate the second tunneling state enough to make detection difficult. With the internal rotation Hamiltonian discussed above with a barrier of 60 cm-I, the pure tunneling splittings of the lowest two states are estimated to be 5 and 1.5 cm-I respectively, for the PF3.Hz0 and PF3-Dz0species. In the limit of free internal rotation, the differences for the two lowest states increases to 15 and 9 cmcl for the HzO and D 2 0 species. We conclude that the combination of a perturbation of the level pattern due to the asymmetric HDO and the depopulation due to lifting of spin restrictions is the likely reason for the absence of the second tunneling component. Some experiments were made to detect the missing levels by using neon as a "nonrelaxing" carrier gas, but they were not successful either. Given that PF3.H20 has C,symmetry with a pair of fluorine and hydrogen atoms straddling the symmetry plane, the location of the H 2 0 around the PF3 was deduced by least-squares fitting of the observed moments of inertia for the three isotopic species combined with dipole moment considerations. Assuming that HzO and PF3do not change their geometry upon c~mplexation,'~ the structure of the complex can be described by the distance between the centers of mass of PF3 and H 2 0 (Rc,,,)and the angles €3' between the C2axis of the H 2 0and the &,vector, and €3, between the PF, symmetry axis and a perpendicular to R,, (Figure 1).
p$) Q;%.&p
3526 The Journal of Physical Chemistry, Vol. 95, No. 9, 19'91
Q; a .. a ..
,,(.._............. %.... ,,(.._............. ...
.....
a.....
........_,..__._ ........_,..__._
"
........... ...........
" "
"
n C: C
LaBarge et al. TABLE VI: Experimental K r a i t c b a Substitution coordinates' for Hydrogen and Dipole Moment Components Compared with Values Calculated for Structures I-IV in Figure 1 obsd I I1 111 IV
la,l/A Ib#A
Ic,l/A lpJ/D Ip,l/D
2.921 0.641 0.287 2.308 0.283
2.974 0.757 0.183 1.41 1 0.593
2.934 0.757 0.694 0.497 0.671
2.934 0.757 0.694 1.263 2.583
2.973 0.757 0.183 0.625 2.497
Single substitution coordinates. The double-substitution coordinates ( D 2 0 data) are (a,) = 2.993, b, = 0.633, c, = 0.0 (c: was negative).
Figure 1. Four structures obtained by fitting the moments of inertia of the three isotopic species with the definitions of the internal coordinates as indicated. R, = 3.250 (2) A for all structures. Values obtained for the angles are as follows: I, 8, = 62O, e2= 28O; 11, el = 57O, 8, = -27O; HI, el = -570, e2= 27'; IV,e, = -620, e2= -28'. Only structure I is compatible with the hydrogen substitution coordinates and the dipole moment components.
The orientation of the PF, about its local C,axis is indeterminable from the moments of inertia, and a rotation of 60' will also fit all the inertial data. The orientation in the figure is preferred since the other possibility will place the oxygen and fluorine atom in the symmetry plane much closer than the sum of their van der Waals radii. From the parallel axis theorem,15 it can be shown that Ib is a function of R , but not dependent on el and el. The latter values are obtained from I, and I,. However, the A rotational constant changes appreciably between the AI and A2 states, indicating that it is most affected by the tunneling motion. Consequently, a least-squares fitting of the moments of inertia (Al state) to R,, e,,and e2was employed which weighted Ib and f, 100 times greater than fa. This gave four fits of similar quality differing in the signs of el and e2with AZ,,(obs-calc) for I, and I, = 0.2 amu Az and for I, ;r 1.60 amu A2. The four structures are labeled I-IV in Figure 1. It can be seen from the values of €I1 that all four structures orient the H 2 0 so that the oxygen is closer to the PF3 and the protons more distant from it; this result is fixed by the deuterium isotopic substitution data, which assign the H atoms a large a coordinate. The PF3 orientation is more ambiguous; two structures (I, 111) tilt the phosphorus end toward the H 2 0 , whereas structures 11 and IV do the opposite. A choice between the four possibilities can be made by comparing the values of the H coordinates and the dipole components predicted for each configuration with the observed Kraitchman substitution coordinatesI6 obtained from the deuterium data and the dipole moment components. The predicted dipole components were obtained from vector addition of the dipole moments for free H 2 0 and PF,." The predicted and observed values are compared in Table VI. The large pc components expected for 111 and IV make these structures unlikely. The appreciable c coordinate for hydrogen expected for I1 and 111 is incompatible with the small isotopic shifts observed for P, for the D 2 0and HDO species. The only configuration compatible with the data is I, where R,, = 3.252 A, el = 62.2O,and e2= 28.1O. The lack of agreement between model I and the Kraitchman calculated coordinates arises from the large-amplitude vibrational motions and the tunneling motion and is not unusual for such complexes. The lack of agreement in the observed and calculated dipole components can arise from two sources: vibrational averaging effects and polarization effects, with the latter probably the more important. The difference of -0.9 D in the pacomponent can be compared to the HZ0.SO2dimer where the observed value for pa is 0.67 D (15) Goldstein. H. Classical Mechanics; Addison Wesley: New York, 1959; p 150. (16) Gordy, W.; Cook, R. L. Microwaue Molecular Spectra; Wiley-Intersciena: New York. 1984; Chapter 13. (17) Nelson, R. D.; Lide, D. R; Maryott, A. A. NSRDS-NBS 1967, 10.
larger than predicted from the monomer units. Since the moments of inertia contain the effects of large amplitude vibrational motions, the structural parameters for the preferred structure I in Figure 1 must be considered only approximate. There is no easy way to estimate how close these values may be to the equilibrium structure but f0.05 A for R,, and f 5 O for el and €I2 should be reasonable estimates. Discussion The structure of the PF3.H20complex is quite different from the PH3 and P(CH3)3 complexes with H X and HCN, which exhibit hydrogen bonds to the phosphorus lone pair as noted in the Introduction. Instead, the structure bears more similarities to the PF,.Ar and S02.H20 complexes. Ab initio calculations2 of the electric field around PF3 have shown that the Ar atom in PF3.Ar is located in a region of high positive potential over a PF2 face, 3.96 A from the center of mass of the PF,. This presumably maximizes the induced dipole moment in the Ar and enhances the interaction energetics. The oxygen atom in PF3-Hz0is located over this same face at a distance of about 3.25 A from the center of mass of the PF,. Because it is smaller, it moves closer in to that face to a point with even a greater positive potential. This enhances the electrostatic interaction between the PF3 and the oxygen end of the water. Turning to the SO2.H20complex, it has a stacked configuration with the water oxygen atom close to the sulfur similar to the oxygen-phosphorus alignment in PF3.H20. The O H and OS bonds straddle a symmetry plane and pseudoeclipse each other with the H 2 0 and SOz planes tipped away from parallel by an angle of 46'. A similar eclipsing arrangement of the O H and P F bonds also occurs in PF3-H20. In this case the H 2 0 and PF2, planes are tipped only 1 4 O from a parallel orientation. In both of these complexes, it is apparent that the monomer dipole moments tend to be opposed leading to small p, Components. In an effort to explore these electrostatic interactions, we attempted to model the overall induced-dipole components, estimated to be pINDa= 0.897 D and pINDC = -0.310D from Table VI. A procedure was employed that has worked reasonably well to calculate the small induced-dipole moments in the rare-gas complexes such as PF3.Ar and PF3-Kr.1,2 A G A U S S I A Na-b~ ~initio calculationI8 ( H F level, 6-31G*)was performed on free PF, to estimate the electric field from the PF3at the position of the center of mass of the H 2 0 molecule in the complex. With this electric field, the polarizability components of H2O,I9 and the proper geometric considerations, the induced pa and pc components from the HzO were estimated to be 0.179 and 0.033 D, respectively. The process was repeated with an ab initio calculation of free HzO to estimate the field a t the site of the PF3 center of mass. This was combined with the polarizability components for PF320to obtain the induced components pa = 0.374 D and p, = -0.121 D. The net induced moments of 0.554D (pa) and -0.088 D (p,) (18) Friach, M.J.; Head-Gordon,M.;Schlegel, H.E.;Raghavachari, K.; Binkley, J. S.;Gonzalez, C.; Der-, D. J.; Fox, D.J.; Whiteside, R. A.; Seeger, R.; Melius, C. F.;,Baker, J.; Martin, R. L.; Kahn, L.R.; Stewart,J. J. P.; Ruder, E. M.;Topiol, S.;Pople, J. A. GAUSSIAN 88; Gaussian, Inc.:
Pittsburgh, PA, 1988.
(19) Murphy, W. F. J . Chem. Phys. 1977,67. 5877. (20) Rhec, C. H.; Metzger, R. M.; Wiygul, F. M. J . Chem. Phys. 1982, 77. 899.
3527
J . Phys. Chem. 1991, 95, 3527-3532
Figure 2. Structural parameters obtained from the spectroscopic data (I) and from ab initio calculations (HF/6-31G*) with geometry opti-
mization for conformations with H atom straddling the symmetry plane (11) and lying in the symmetry plane (111). I1 lies 300 cm-I below Ill, and its binding energy was 1542 cm-l. Total energies: 11, -715.146738 hartrees; Ill, -715.145 396 hartrees. have the correct signs and are in fair quantitative agreement with the values estimated above. The lack of quantitative agreement probably arises from several sources including errors in the polarizability of PF3, errors in the electric field from the a b initio calculation, the assumption that a point polarizability at the center of mass of each species can be used to model their induced-dipole moments and the neglect of vibrational averaging effects. The possibility of some charge transfer also cannot be eliminated. Ab initio methods were also employed to examine the conclusion that the water molecule in PF3-Hz0straddles a symmetry plane as opposed to lying in the plane. For these two conformations, calculations with geometry optimization of R , and the tilt angles were performed at the HF/6-31G* level to estimate the structural parameters associated with the lowest energy for the two con-
formations of H20. These structures and energies are illustrated in Figure 2. The H 2 0 out-of-plane conformation (11) is placed -300 cm-l below the H20 in-plane conformation (111) and 1540 cm-l below dissociation and replicates the experimental R , and in I to -0.1 A and -So. However the value for O2 is different by 14'. It has been the experience in our laboratory and verified by recent work on 03.H20'zand the N20-HX systemz2 that calculations at the Hartree-Fock level will often correctly distinguish between conformational choices; it is gratifying that the PF3.Hz0comparison follows this trend. Nevertheless some caution is warranted since it is clear that calculations beyond the HF level must be made, especially when the complexes are quite weak, to reasonably reproduce experimental structures and estimate energy differences between isomers and binding energies. A rough estimate of the energetics of complex formation can be obtained by using Millen's modified pseudodiatomic approximation. In this model,21the centrifugal distortion constant 0, is related to a force constant for stretching of a hypothetical van der Waals bond between the PF3 and HzO, which gives k, = 0.073 mdyn/A. With a Lennard-Jones 6-12 potential, the force constant is related to a well depth by e = k,Rm2/72,resulting in t = 542 cm-I. This can be compared with the values in PF,.Ar and H20-SOZof 0.016 mdyn/& 180 cm-' and 0.084 mdyn/A, 520 cm-' respectively, when the analyses are done in a consistent manner. Clearly, the association of H 2 0 with either of the polar species is a considerably stronger interaction than in PF3-Ar where only dispersion and an induced polarization of the argon contribute to the binding. It is also apparent that considerably larger polarization occurs in the water complexes, as discussed above. Acknowledgment. This work was supported by Grants CHEM86 14340 and CHEM89 17945 from the National Science Foundation, Washington, DC. We are grateful to Dr. C. W. Gillies for communicating his results on 03-H20prior to publication. (21) Millen, D. Can. J. Chem. 1985,63, 1477. Equation 21, which strictly holds only when the symmetry axes of the HzOand PF, are perpendicular to &,Le., 8 , = 90°, Oz = Oo, was used, resulting in b = B D / A H ~ BD/BpF, and c C D / B H 0 + CD/CpF,. (22) Zeng, P.; Sharpe, S. W.; Reifschneider, D.; Wittig, C.; Beaudet, R. A. J. Chem. Phys. 1990, 93, 183.
+
k7.
Polarized FT- I R Spectra of Water in Dodecylpropyldimethyiammonium Bromide Hemihydrate Noriyuki Kimura Institute for Chemical Research, Kyoto University, Uji, Kyoto-Fu 61 1. Japan (Received: September 25, 1990) Polarized infrared spectra of two kinds of specimens, with the (001) and (1 10) surface planes, of dodecylpropyldimethylammonium bromide hemihydrate (DPDMJ/,H20), deuterated hemihydrate (DPDMJ/2Dz0), and isotopically diluted hemihydrate (DPDM.1/2(958 D 2 0 + 5% H,O)), were observed in the frequency region from 5200 to 250 cm-I. The electric vector of the normally incident polarized radiation was rotated at 10' interval on the crystal planes. From the polychroism observed, the intra- and intermolecular vibration bands of water were classified into three infrared active species (A], B,, and B2). Furthermore, from the band frequency and the isotopic frequency ratios, these bands were assigned to the three fundamental vibrations, two librations, and their overtones and combinations. The directions of the transition moments of the water bands and of the CH2 stretching and scissoring bands of DPDM accord well with those expected from the crystal structure. The OH and OD stretching bands of HOD in isotopically diluted hemihydrate was found to split into two components. This fact was interpreted as due to the presence of the two kinds of configurations, Brl.-HOD-.Br2 and Br,--DOH--Br,, in the crystal, suggesting that the C, symmetry around water was slightly distorted in the crystal.
Introduction The structure and aggregation properties of alkylammonium bromidewater systems have attracted much attention because they exhibit a variety of micellar size and shape depending upon the number and length of the alkyl chains. For example, Kunitake et a1.l have reported that dialkyldimethylammonium bromides
with the longer alkyl chains than the dodecyl group form a well-defined vesicular structure which resembles that of phospholipid h " e s in aqueous solution. Zana et ala2have pointed (1) Kunitake, T.; Okahata, Y.;Tamaki, K.; Kumamaru, F.; Takayanagi, M. Chem. h t r . 1977, 387.
0022-3654/9l/2095-3527%02.50/0 0 1991 American Chemical Society
