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J . Phys. Chem. 1989, 93, 1586-1588
Ionic Hydrogen Bond and Ion Solvation. 8. RS-.-HOR with Acidities
Bond Strengths. Correlation
L. Wayne Seck* and Michael Meot-Ner (Mautner) Chemical Kinetics Division, Center for Chemical Physics, National Bureau of Standards, Gaithersburg, Maryland 20899 (Received: May 20, 1988)
The stabilities of RS-eHOR association ions have been investigated by pulsed high-pressure mass spectrometry. Equilibrium constants were determined as a function of temperature in order to define the AHo and ASo values for solvation. Dissociation energies, AHDO, were found to decrease as the difference between the acidities (AHoacid) of the two molecular components increased. For C6H5S-.HOR complexes, a linear correlation of the form AHDO = 22.1 - O.20APHoacid was obtained, while for complexes incorporating HS- or CH,S- as core ions, the expression was AHDO = 19.9 - O.15AAHoadd,both in kcal/mol. Bonds involving CF3CH20Hdeviated substantially from the correlations, suggesting multiple interactions. The successive hydration of HS- and CH3S- was also investigated, and the AHDovalues found for the respective monohydrates, 14.2 and 15.0 kcal/mol, are in excellent agreement with recent ab initio calculations.
Introduction Strong interactions between anions and neutral molecules play a central role in ion solvation and are also important in environments ranging from biological systems to planetary atmospheres. Under ionizing or basic conditions in these environments, sulfur-containing compounds can lead to the formation of sulfide ions, which can participate in strong hydrogen bonding. In a series of articles from this laboratory' we have found, for a variety of BH+-.A complexes, that an inverse linear relation appears to exist between the dissociation energy of BH+--A, AHDO, and the difference between the proton affinities of the two partners; Le., AHDO = a - bAPA, where APA = PA(B) - PA(A). Similar correlations are also found for negatively bound systems, B---HA.2,3 In this case the analogous equation is AHDO = a - bAAHoa,,d,where AAHoacidis the difference between the gas-phase acidities of the two partners (AHoa&jof BH defined as the proton affinity of B-). Qualitatively, these correlations, first observed by Yamdagni and Kebarle? result from the dependence of the efficiency of partial proton transfer between A and B on the relative proton affinities of the components. They generally predict A H D O to within 1-2 kcal/mol, which is extremely useful for estimating bond energies which have not been measured, or those associated with inaccessible environments, such as biosystems. In the present study we have attempted to develop correlations for association ions in which the bonding is of the type RS-e-H0-R. These studies were extended to include measurements of the thermochemistry of multiple hydration involving the core ions HS- and CH3S-. Experimental Section All experiments were carried out with the NBS pulsed electron beam mass spectrometer, which has been described e l ~ e w h e r e . ~ Negative ions were generated by the addition of small quantities of N 2 0or C2HSN02to the reaction mixture. The primary anions, formed by dissociative electron capture (0-or NO,-), reacted rapidly with the sample components to yield the RS- ions of interest. Methane was used as the bulk carrier gas at total pressures between 0.5 and 2.5 Torr. For all equilibria, the usual checks6 were performed to ensure that the measured value of K was independent of total source pressure and the partial pressure ( I ) (a) Meot-Ner (Mautner), M. J . Am. Chem. SOC.1984,106, 1257. (b) Speller, C. V.; Meot-Ner (Mautner) M. J . Phys. Chem. 1985, 89, 5217. (c) Meot-Ner (Mautner), M.; Sieck, L. W. J . Phys. Chem. 1985, 89, 5222. (2) Meot-Ner (Mautner), M.; Sieck, L. W. J . Am. Chem. SOC.1986, 108, 7525. (3) Meot-Ner (Mautner), M. J. A m . Chem. SOC.,in press. (4) Yamdagni, R.; Kebarle, P. J . A m . Chem. SOC.1973, 95, 3504. (5) Sfeck, L. W.; Meot-Ner (Mautner), M. J . Phys. Chem. 1984,88,5324. (6) Sieck, L. W. J. Phys. Chem. 1985, 89, 5552. (7) Lias, S.G.;Liebman, J.; Bartmess, J.; Holmes, J.; Levin, R. J. Chem. Phys. Re$ Data, in press.
TABLE I: Thermochemistry"of Cluster Dissociation and Related Parameters for RS-mHOR Comolexes RSHOR MD' UD' (AHD')~ AAH',ddc C6HsSEtC02H 20.0 (0.4) 25.6 (0.9) 22 9.1 HSCF,CH,OH 26.8 (0.51 22.6 (1.0) 22 10.1 20.3 (o.ij 26.2 (0.3j 21 10.5 17.1 (0.2) 16.8 (0.3) 21.0 (0.2) 16.3 (0.1) 15.0 (0.2) 14.6 (0.1) 15.0 (0.1) 14.2 (0.2) 13.4 (0.1) 11.4 (0.2) 13.5 (0.4) 11.1 (0.1) 9.6 (0.4) 12.6 (0.3) 11.7 (0.5)
23.1 (0.6) 19.9 (0.9) 25.1 (0.4) 19.0 (0.3) 21.2 (0.5) 25.0 (0.4) 26.0 (0.3) 18.7 (0.4) 23.0 (0.2) 19.6 (0.5) 23.5 (1.2) 20.5 (0.5) 19.2 (1.6) 20.4 (0.9) 23.5 (1.7)
20 19 19 18 15 15 15 14 14 12
17.5 22.6 24.0 25.4 32.1 36.5 37.3 38.6 42.4 52.5
"AHD' in kcal/mol, &SDoin cal/(mol deg), T in K; standard state is 1 atm. The error limits given for A H D O and UDo are the standard deviations of the slopes and intercepts obtained from linear regression analyses of the data used to construct the individual van't Hoff plots. 6Values calculated from eq 3, using Pauling electronegativities. CAHoadd values taken from ref 7 except that for C6HSSH. In this case we have taken A/?o,dd(C6HSSH) as 338.2 kcal/mol, which is based on unpublished data from this laboratory.
of the ligand. One association ion designated for study, C6H5S-.HC02H: did not yield a reliable van't Hoff plot. This particular pair was complicated by the rapid switching reaction C6H5S--HC 0 2 H H C 0 2 H H C 0 p H C 0 2 H C&SH, which is driven by the unusually high bond strength in H C O p H C 0 2 H . ' However, a AG of -9.6 (f0.3) kcal/mol was obtained for the equilibrium C6HsS- H C 0 2 H C6HSS--HC02Has a result of multiple experiments at a single temperature (488 K). The C6HSS-.Hz0association ion has a mass coincidence with I- (m/! 127), which is a background impurity ion of unknown origin in our instrument, especially in reaction mixtures containing water as a component. Consequently, this particular system was studied by using D 2 0 to avoid mass overlap. In spite of pretreatment of the ion source and inlet system with high flow rates of neat D 2 0 for prolonged periods of time, the ion C6H5S-.HD0 was found to gradually appear in reaction mixtures containing C6HsSH, D20, and CHI, and dominated the product ion mass spectrum after a period of only 20-30 min. Because of the unknown contribution from C6HSS-.Hz0 to the m / z 127-129 manifold under these conditions due to background I-, we were unable to carry out typical temperature study, which usually involves a single mixture studied at various temperatures and total pressures over a period of several hours followed by replicate measurements at different concentrations. Therefore, the equilibrium C6H5S-
+
-
+
+
-
This article not subject to U S . Copyright. Published 1989 by the American Chemical Society
Ionic Hydrogen Bond and Ion Solvation
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1587 22 T
AH; 16 18
In K
CH~S p m A \
c
ll
-
9
5-
-
1 I I
-7 I
l
l
l
l
l
l
l
l
l
l
l
l
l
l
\ I
IO
I 1 1 1
I
20
I
1 I 1
I
30
I I I I
I1
40
I 1 I
I
50
I I I 1
i
AAHk
l
Figure 3. Binding energies (AH,') for various RS-mHOR complexes vs differences in gas-phase acidities (AAHo,"d) between RSH and HOR. All values in kcal/mol.
nations were studied; C6H5S-.HOCH2CF3and HS-.HOCH2CF3. On the basis of relative acidities ( A M a d , Table I), eq 1 predicts a AHDO for C6H5S-.HOCH2CF3, of 17.2 kcal/mol and eq 2 predicts 18.4 kcal/mol for HS-.HOCHzCF3 both of which are below the respective experimental values of 21.0 and 26.8 kcal/mol and outside of error limits. The increased stabilities of these particular complexes can be most reasonably ascribed to the formation of a second partial hydrogen bond involving the C-H+ dipoles in the methylene group (structure 1A). The C-H+ bond
/
F
\
F
F
1A 1
1
2.4
1
1
1
27
1
l
I
1
1
30
1
3.3
1
1
1
3.6
1
1
1
1
3.9
1
I
l
l
4.2
1 OYT
Figure 2. van't Hoff plots for the hydration of HS- and CH3S-. Arrows indicate alternate scale of 1/T values for these particular plots.
+
-
D 2 0 C6H5S-.Dz0 was investigated at only two temperatures, 308 and 363 K, with the equilibrium constant evaluated immediately after sample makeup at various concentrations of DzO. Results and Discussion The van't Hoff plots for the various association equilibria are shown in Figures 1 and 2, and the results are summarized in Table I. The table is ordered according to increasing AAHoaddbetween the two neutral partners from which the association ion is derived (RSH and ROH). The error limits given for AH,' and A S D O are the standard deviations of the slopes and intercepts obtained from linear regression analyses of the data used to construct the individual van't Hoff plots. As in previous studies, the combined thermochemical data again indicate an inverse relationship between AHDO and AAHoacid.Figure 3 shows that the results may be divided into two separate series, one for the complexes of C6HSS-and a second, with a slightly depressed slope, for those incorporating HS- and CH&. The appropriate correlation parameters are given in eq l and 2 (in kcal/mol).
C~HSS-...HO AHDO = (22.1 f 0.2) - (0.20 f O.O1)AAHoa,id
(1)
6 points, correlation coefficient = 0.997 HS-, CH3S-***HO AHDO = (19.9 f 0.3) - (0.15 f O.Ol)AAHoacid
10
1c
dipole strengths are expected to be higher in C F 3 C H 2 0 Hthan in, for example, CH3CHz0Hdue to increased electron withdrawal by the CF, group relative to CH3. Similar CH+.-X- bonds are found in complexes of halide ions with CH3CN and other aprotic ligand^*^^ and have been invokedl0 to explain the enhanced stabilities of anionic association ions incorporating HCO2H, where multiple hydrogen bonding is also possible (structure 1B). Although space-filling models indicate that head-and-tail configurations such as 1C cannot be ruled out, these bonding geometries severely restrict rotations in both the ion and the ligand. The thermochemistry of C6HSS-.HC0zHcould not be evaluated quantitatively. Combining the measured AGO of -9.6 kcal/mol for the association equilibrium at 488 K with an assumed ASD' value of 26 eu for the complex ion (observed for C~HSS-*CH$OzH and C6H5S--C2HSC0zH bonds, Table I) yields a A H D O for C6H5S-.HCO2H of 22.3 kcal/mol. If a higher value of A S D O is appropriate for this complex, which is usually the case when multiple bonding restricts internal rotations, there is a corresponding increase in A H D O of 0.5 kcal/mol per entropy unit value increment above 26 cal/(deg mol). Taking the AAHoacid for C6HSS-.HCO2H as 6.9 kcal/mol, a A H D O of only 20.7 f 0.2 kcal/mol is predicted for this complex by using eq 1, which is significantly below that suggested by the preceding treatment of AGO. Therefore, it appears that multiple bonding may increase the stability of C6H5S-.HCO2Hrelative to analogous complexes incorporating other carboxylic acids, although more data are required for a variety of B-.HC02H and B-.RC02H combinations before it can be verified that HCOzH has special anionic chelating properties. Larson and McMahon" have suggested an empirical relationship which may be used to estimate bond strengths in anionic
(2)
5 points, correlation coefficient = 0.989
Ions incorporating CF3CH20Has the ligand showed substantial positive deviations from the correlation lines. Two such combi-
(8) Yamdagni, R. Kebarle, P. J. Am. Chem. SOC.1972, 94, 2940. (9) Magnera, T. F.; Caldwell, G.;Sunner, J.; Ikuta, S.; Kebarle, P. J . Am. Chem. SOC.1984, 106, 6140. (10) Caldwell, G.; Kebarle, P. J . Am. Chem. SOC.1984, 106, 967. ( 1 1) Larson, J. W.; McMahon, T. B. J . Am. Chem. SOC.,in press.
J . Phys. Chem. 1989, 93, 1588-1592
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association ions. Their equation is of the form AHDO(B-*.*HA)=
in kcal/mol, where x(A) and x(B) are the electronegativities of the respective heteroatoms in each partner within the complex. Application of the expression to bonds of the type S---HO, where S- is incorporated in C6H5S-, CH3S-, and HS-, yields a single correlation line represented by AHDo(S-.*.HO) = 23.9 0.234AAHoacid(in kcal/mol). Although the data seem to show separate correlations for the C6HSS- and the CH3S-, HSgroupings, with C6HSS-bonds exhibiting a slightly higher slope and intercept (Figure 3), the empirical relationship does reproduce the experimental bond strengths for C6HSS-complexes quite well, especially at higher values of AAHoaed,where A H D O is depressed. The experimental A H D O values for the monohydrates of HS- and CH3S- are almost identical with those predicted within measurement error, indicating the potential utility of eq 3. The individual values of A H D O derived from this relationship for the various complexes are included in Table I for comparison with the present results. Recently we established2 a correlation line for RO-...HO bonds of the form AHDO = 27.5 - 0.29AAHoaCid.Comparison of these parameters with those describing RS---HO interactions (eq 1 and 2) indicates that complexes incorporating 0- are more stable by 2-7 kcal/mol at AAHoacidvalues below 40 kcal/mol than those incorporating S-. The decreased stabilities for complexes having second-row elements at the site of ligand attachment have been considered in detail by Gao et a1.,12who performed high-level ab
initio M O calculations for a variety of anionic hydrates. They concluded that several factors are operative in weakening the bond strengths, including increased ionic radii and diminished covalent character of the hydrogen bond. These factors combine to increase the bond lengths, which were found to be -2.5 A for RS-sHOH complexes compared to only 1.5-1.6 A for those incorporating RO-eHOH bonds. Their calculated A H D O values for HS-eHOH and CH3S-.HOH of 15.6 and 15.4 kcal/mol are in very good agreement with the respective experimental values of 14.2 and 15.0 kcal/mol found for these combinations. The A H D O values for the stepwise addition of H 2 0 to HS- and CH3S- are given in Table I. Consistent with the trends found for other core ions,13A H D O decreases rather rapidly upon successive hydration up to n = 3, at which point the values are within 1 kcal/mol of AHovap(H20), 10.5 kcal/mol. Therefore the weak ionic contributions to solvation are effectively dissipated within the first 2-3 water molecules. Additional stepwise addition should proceed with bond energies near 10.5 kcal/mol, which we found for CH3S--(H20)4,9.6 kcal/mol, equal to the neutral condensation limit within experimental error. Based on criteria suggested earlier,I3there is no evidence for shell-fillingeffects in these weakly bound systems. Acknowledgment. This research was supported by the Office of Basic Energy Sciences, United States Department of Energy. Registry No. C6HSS-,13133-62-5;HS-, 15035-72-0;CH$, 1730263-5; EtC02H, 79-09-4; CF3CH20H,75-89-8; MeC02H, 64-19-7; iPrOH, 67-63-0; t-BuOH, 75-65-0; EtOH, 64-17-5; HOH, 7732-18-5; MeOH, 67-56-1. (12) Gao, J.; Garner, D. S.; Jorgensen, W. L. J . Am. Chem. SOC.1986,
108, 4784.
(13) Meot-Ner (Mautner), M.; Speller, C. V. J. Phys. Chem. 1986, 90,
6616.
Force-Field Calculations Giving Accurate Conformation, AH,' ( T ) , S o( T ) , and C, O ( T ) for Unsaturated Acyclic and Cyclic Hydrocarbons Terry G. Lenz**+and John D. Vaughan*,* Department of Chemical Engineering and Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 (Received: June 7 , 1988)
A modified version of the Boyd MOLBD3 force-field method has been employed to compute the structure, standard enthalpy of formation, and standard absolute entropy for a variety of dienes and unsaturated ring and methylene-bridged compounds. The modifications to M O L B D ~included incorporation of -ene and -diene parameters suggested by Anet and Yavari, as well as our adjustment of C(sp2)-C(sp2)-C(sp2) and C(sp2)-C(spz)-C(sp3) bond angle parameters for unsaturated five-member rings and methylene-bridged compounds. Our MOLBD3 results for 1,3-butadiene, 1,3-pentadiene, 1,3-~yclohexadiene,1,3cycloheptadiene, cyclopentene, norbornane, norbornene, norbornadiene, bicyclo[3.2.lloctane, bicyclo[3.3.llnonane, and 1,3-~yclopentadieneare generally in excellent agreement (typically f 1 kcal/mol for AH?,and f l cal/(mol.K) for S o )with observed values and with calculations we and others have made with the Allinger MMPZ and Ermer-Lifson CFF-3 programs. These molecular mechanics methods are thus capable of predicting thermochemical properties with sufficient accuracy for useful thermodynamic calculations.
Introduction At present there are powerful empirical and semiempirical methods available for calculating structural and thermodynamic properties of isolated molecules. Two particular methods that have received considerable attention during the past two decades are the force-field (molecular mechanic^)'^^^ and molecular orbital methods (such as MINDO, MNDO, and AM1).2 Such computational methods provide a basis for accurate calculation of molecular geometries, vibrational frequencies, and thermodynamic 'Department of Chemical Engineering. *Department of Chemistry.
0022-3654/89/2093- 1588$01.50/0
properties for a wide range of molecules having nontrivial structures. Molecular mechanics methods in particular have proven capable of computing standard enthalpies of formation, AH?,agreeing to within &1 kcal/mol of experimental values and (1) (a) Burkert, U.; Allinger, N. L. Molecular Mechanics; ACS Monograph 177; American Chemical Society: Washington, D.C., 1982. (b) The M M P ~program employed in our studies was obtained from the Quantum Chemistry Program Exchange, Indiana University. (2) (a) Dewar, M. J. S.; Ford, G. P. J . Am. Chem. SOC.1977, 99, 7822. (b) Dewar, M. J. S.; Storch, D. M. J . Am. Chem. Soc. 1985, 107, 3898. (c) Dewar, M. J. S.; Zoebisch, E. G.;Healy, E. F.; Stewart, J. J. P. J . Am. Chem. SOC.1985, 107, 3902.
0 1989 American Chemical Society
