J. Phys. Chem. 1991, 95,7617-7621 and [HNC] may be interpreted as evidence for their formation via ion-molecule chemistry involving HCNH+ and H2NC+ and the lack of isomerization of HNC to HCN at the very low densities prevailing in the interstellar medium. The results of this study have implications for the production of HCN on Titan. Current theories speculate that HCN can be p r o d u d on Titan by ion chemistry9 and neutral chemistry,' The ion chemistry involves the following series of reactions: N+ + CHI+ HCN' + H2 H (1 1) HCN" HCNH+
+ H2
+e
-
+ HCNH' + H H C N (HNC) + H
+
(12)
(10)
The neutral chemistry involves the following reactions:l N + CH3 HZCN + H +
H
+ H2CN
HCN
+ H2
(2) On the basis of the present study, we suggest that the neutral and +
7617
ion chemistry may be coupled in the upper atmosphere of Titan by the action of vacuum ultraviolet solar radiation on H2CN resulting in autoionization of H2CN to HCNH'.
Acknowledgment. We are grateful to Prof. G. Barney Ellison for helpful discussions and for communicating the data On the electron affinity and heat Of formation Of HzCN prior to publication; we thank Dr. William H. Kirchhoff for making us aware of the work of Prof. Ellison and his co-workers. We acknowledge helpful discussions with Drs. Sharon Lias, Marylin Jacox, and Joel Liebman, Prof. David Turner, and especially Dr. Morris Krauss. G.M. thanks the National Academy of Science for the award of a Research Associate. F.L.N. acknowledges support under NASA Grant NSG-5173 to the Catholic University of America. The work at GSFC was supported by the NASA Planetary Atmospheres Program. The work a t BNL was supported by the Division of Chemical Sciences, US.Department of Energy, Washington, DC, under Contract No. AC02-76CHOOO16.
Nature of Hydrogen Bonds Formed by Simple Amides and Sulfonic Acids in Inert Solvent. 1. 'H NMR Low-Temperature Studles Marek Ilczyszynt Dzpartement de chimie. Universite de Montreal, C.P. 6128, Succ. A, Montreal, Quebec, Canada H3C 357 (Received: December 27, 1990)
For the first time direct spectroscopic evidence was found that amides (B) are the 0 and N bases. *HNMR low-temptrature investigations show that sulfonic acids (AH) form with amides AH-O(B)N and AH-O(B)N-HA hydrogen-bonded complexes. The latter can exist in two different tautomeric states with exchange between them governed by the electron structure rearrangement of the complexed amide. The O H 0 proton in the AH-O(B)N complexes probably occupies only one position inside the hydrogen bridge (one tautomeric state of the complex) which is sensitive to the acid-base interaction strength and to the solvation. These phenomena are monitored by its chemical shifts.
Introduction Hydrogen-bonded complexes formed by trifluoromethanesulfonic acid (CF3S03H), methanesulfonic acid (CH3S03H),and ptoluenesulfonic acid (C7H,S03H) with NJVdimethylformamide (DMF) and NJV-dimethylacetamide (DMA) in dichloromethane solutions were investigated. CF3S03H is known for its interesting properties,' great ability to hydrogen bond with water,2 and self-a~sociation.~It is one of the strongest acids and is frequently used in different protonation reactions.c8 To my knowledge there is only one paper2 dealing with its behavior as the acid in an aprotic and inert solvent. D M F and DMA are usually treated as the oxygen bases, but the nitrogen is sometimes taken into account as the additional basic center.*I2 Evidences for the N-protonation are drawn mainly from kinetic investigation^'^-'^ and are supported by quantum mechanical calculations.I6 S ectroscopic results for crystal^'^ and strongly acidic solutions1g-20prove the interactions to the oxygen atom only whereas the results obtained for the less active media are controversial or provide indirect evidences for the N interaction^.'^+^*^^ Experimental Section
CF3S03H (Aldrich), CH3S03H (Fluka), D M F (ACP), and DMA (Aldrich) were distilled under reduced pressure in a dry nitrogen atmosphere. C7H7S03Hmonohydrate (Aldrich) was dried under vacuum at 50 OC for 190 h; the final pressure was 'Present address: Institute of Chemistry, University of Wroclaw, ul Joliot-Curie 14. 50-383Wroclaw, Poland.
2), the O H 0 proton is more shielded as a result of its distance shortening to the amide oxygen. This is demonstrated by the spinspin coupling between the O H 0 and C-H protons in the CF3S03H-DMF complex. The corresponding coupling with C-CH, protons in the CF3S03H-DMA complex is too weak to be recorded.l* (26) BBhner, U.;Zundel, G . J . Phys. Chem. 1985,89. 1408. (27) Arnett, E. M.; Mitchell, E.J.; Murty, T. S. S. R.J . Am. Chem. Soc. 1974, 96. 3875. (28) Guthrie, J. P. Can. J . Chem. 1978, 56, 2342. (29) It is interesting that the NMR results for the CHJn0,H and C7H,SO,H complexes (Table I) support the surprising finding that pK, values of these acids are very similar. (30) Lau, K. F.; Vaughan, R. W . Chem. Phys. Lett. 1975, 33, 550. (31) Appleton, Q.;Bernander, L.; Olofsson, G. Tetruhedron 1971, 27, 5921.
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7619
H Bonds Formed by Amides and Sulfonic Acids. 1
,q 18.4
17.6
73.6
h
1
0.
D
b
0
1
2
3
5
4
6ApK.
12'8
12.010
14
18
2.2
2.6
3.0
3.4
3
8
~
Figure 4. Acidic proton chemical shifts of CH3S03H+ DMF in dichloromethane solutions at -120 OC vs the molar ratio of the acid to the
base.
Figure 5. Bridging O H 0 proton chemical shifts of the 1:l and 2 1 sulfonic acid-amide complexts vs pK, difference between the protonated base and the acid.
TABLE II: Proton Chemical Shifts for a Sample of 0.21 M CF@O,H and 0.09 M DMF in Dichloromethane Solution at Different Temwratures
The full 0-protonation of the amides, when the ion pairs without H bonds are formed, is manifested by an additional, dramatic shielding (9.4-9.8 ppm) of the acidic proton as it was shown earlier.'**3' The equilibrium A-***HO(B)N+e A- HO(B)N+ (1) is negligible for similar complexes dissolved in acetonitrile26and probably for the sulfonic acid amide solutions because the sharp A signals are recorded for the CF3S03Hcomplexes (the most polar H bonds studied here) below -100 "C and for R < 2.4. The situation can be different for R > 2.4 where solubility of the CF3S03H complexes is reduced. OSdvation of the AH-O(B)N Complexes. The R dependences of bA and 61,below -100 OC (Figures 1-4) indicate interactions between the AH-O(B)N complexes and the excess of the acid. There are two probable sites of the solvation, the amide nitrogen and the complex SO3group, which are easy to distinguish because they should exhibit opposite cooperative effect^.^*-'^ It can be expected that in the last case AH-O(B)N + AH F? AH-AH-O(B)N (2) the additional H bond (AHA) will increase the strength of the principal bond (AHB) and the shielding of its O H 0 proton. Additionally, the AHA proton will probably shift from an edge position, AH--A-*H-.B, toward the more central and deshielded one, A--H-.A--.HB+, when the strength of the interaction to B increases. The positions of the A and D signals below -100 "C and for R = 2 (Figures 1-4) depend on ApKa according to the above predictions and can be referred to as the AH-AH-B and AHAH-B protons, respectively (Figure 5 , broken lines). The association to the complex SO3 groups resembles the sulfonic acid self-a~sociation~~.~ and the interactions in the crystal CF3S03H3' and in carboxylic acid-base-inert solvent systems.3s It is interesting to note that the screening variations of the AHB proton, when R = 1 R = 2, are the highest for its central position in the bridge (Table I, Figure S), thus confirming the high polarizability of this kind of H bond.26 Character of the AH-O(B)N Hydrogen Bond. 'H NMR lowtemperature results for the sulfonic acid + amide systems in
+
+
-
(32) Frank, H. S.;Wen, W.-Y. Discuss. Faraday Soc. 1957, 24, 133. (33) Kleeberg, H.; Klein, D.; Luck, W. A. P. J . Phys. Chem. 1987, 91, 3201. (34) Ye. Z.; Yazdani, S.;Thomas, R.; Walker, G.; White, D.; Scott, R. M. J . Mol. Strucr. 1988, 177, 513. (35) Zundel, G.; Metzger, H.; Scheuing. I. 2.Narurforsch. B 1967. 22, 127. (36) KompwhulteScheuing,1.; Zundel, G. J . Phys. Chem. 1970,74,2363. (37) Bartmann, K.: Mmtz, D. Acta Crystallogr. 1990, C46, 319. (38) Arnett, E. M.;Chawla, B. J . Am. Chem. SOC.1978, 100. 217.
the acidic protons
N.N-dimethylformamide protons
~nB. 1,
O C
symbol
IO -70
symbol
kno.
13.25
D A" A A'
-100
ppm
D A4
13.93 13.54 13.11 12.30 14.30 13.20 13.14
ppm
symbol
8.32
B
B'
8.37 8.15
C C'
BO
8.42 8.38
C
~N(CH,),?
ppm
3.39 3.28 3.41 3.29 3.18 3.06 3.44 3.3 I
"Doublets; signal A is better resolved.
dichloromethane are consistent with Zundel and BBhner's?6 conclusions from IR investigations of CH3S03Hwith different oxygen bases in acetonitrile at room temperature. In these O H 0 systems the H-bridge proton potential can be presented as a single-minimum well. For the complexes characterized by ApK, from the 0-1 range this minimum appears near the center of the H bond and is probably flat and broad. With an increase of ApK,, the potential well becomes more narrow and its position is shifted toward the base oxygen.26 This explains the ApK, dependence of aAHB (Figure 5 , solid line) and behavior of the central O H 0 signal: (i) the high response on the 0-solvation (Table I) and (ii) its broadening (up to -20 Hz at the lowest temperatures) contrary to the CF3S03H complexes. The signal broadening can reflect oscillations of the O H 0 proton inside the flat potential minimum. In some CHN, NHN, or OHN H bonds these oscillations occur between two distinct positions as the tautomeric equilibrium XH...Y 2 X-...HY+ (3) and a very broad H-bridge signal or even splitted ones are recorded in the 'H NMR Amides as the Oxygen and Nitrogen Bases. In the 10 to -120 "C temperature range the CH3S03H amide systems (R > 1) are represented by (i) the averaged acidic signal (10 to -100 "C) or A,Ddoublet (-100 to -120 "C), (ii) the formyl and C-CH3
+
(39) Denisov. G. S.;Bureiko. S. F.; Golubev, N. S.; Tokhadze, K. G. In Molecular Interacriom; Vol. 2; Ratajczak, H., Orville-Thomas,W. J., Us.;
Wiley: New York, 1990. (40)Denisov, G. S.;Golubev, N. S.J . Mol. Srrucr. 1981, 75, 311. (41) Ilczyszyn, M.; Ratajczak, H.; Ladd, J. A. Chem. Phys. Lurt. 1988. 153, 385. (42) Ilczyszyn, M.; Ratajczak, H.; Ladd, J. A. J . Mol. Struct. 1989, 198, 499. (43) Rospcnk. M.;Sobczyk, L. Magn. Reson. Chem. 1989, 27, 445.
Ilczyszyn
1620 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
TABLE III: Expected Contributions of Different Complexes to the Acidic and Formyl Proton Signals in CFgOJ4 + DMF in Dichloromethane Solutions Wben tbe Molsr Rstio of tbe Acid to tbe Base Is Higher tban One at -70 OC
sienal
comolex
note
D
a
A“ A
Y‘
with 0-solvation for 2 < R 5 3.3 with and without 0-solvation
~
I
a Y Y’ Y 7’
46h A’
1
1 14
1.8
2.2
2.6
0 3.0
3.4
B
1
R
Figure 6. Acidic proton chemical shifts of CF3S03H+ DMF in dichloromethane solutions at -70 O C vs the molar ratio of the acid to the base.
singlets, and (iii) the N-(CH3)2 doublet. Different results were obtained for the CF3S03H + amide solutions (R > 1) at moderately low temperatures: -70 f 15 and -50 f 15 O C for DMF and DMA, respectively. Detailed analysis for the CF3S03H D M F solutions is presented below; the C7H7S03H amide systems were not systematically investigated. At -70 “ C and for R = 2.31 (Table 11) there are four broad signals in the acidic proton region (A, A’, A”, D), two singlets in the formyl proton region (B, B’), and two doublets in the methyl proton region (C, C’). C’, contrary to C, is broad and appears near the coalescence point with the same separation between the two components of the doublet as that in C. Similar observations were made for solutions with 1.7 I R 5 3.3 (Figure 6). When R is out of this range, the solubility is insufficient for the investigations. DMF, as other amides, can exist in two different tautomeric states schematically presented below:12
-
-
+
+
7
y
7
3
S+
S-OL’C’”
I
a
3
O=C-N
(4)
I
CH3
(DMF)
y
for 1.7 5 R
with and without 0-solvation
a Y 7’
B‘
+
where C, I (Cy Cy,).” These considerations can be summarized by presentation of the associates formed when R = 2 AH-AH-O(DMF)N AH-O(DMF)N-HA
AH-O(DMF)’N-HA
+
where C, = (C, C,,) and (number of O H 0 bonds):(number of O H N bonds) = ( I D + IA):IAt = 3. For R > 2 the new signal (A”) is recorded between A and D (Figure 6), IA,:IB:IB, approaches the ratio 1:1:1 when R 3.3, and the intensity ratio between the overlapped D, A”, A signals and A’ signal changes from -3.2 to -5.6 when R increases from 2.3 to 3.3. One can conclude that (i) y-complexes convert into y’ones with A” signals representing their OH0 bonds (Table 111) and (ii) approximately equal amounts of DMF molecules exist in the a and (y ~t y’) states solvated by the acid molecules: IAt:IB:IB, 1:1:1 (for R 3.3)
-
-.
(Cy
-
These polar and unpolar structures are obviously represented by the B, C and B’, C’ signals, respectively. The sharp C and the broad C’ signals describe different rotation around the N - C bond: slow on the N M R time scale for (DMF) and relatively fast for (DMF)’. The formyl and methyl signals are partially overlapped, but it seems that their relative intensities are the same and decrease with R: I B : I ~=, IC&, = -8, -3, and 1.0 for R = 2.1, 2.3, and 3.3, respectively. Different kinds of the complexes can be postulated for DMF (and other amides): AH-O(DMF)N, a ; O(DMF)’N-HA, 8; AH-O(DMF)N-HA, y; AH-O(DMF)’N-HA, y’; where 0complexes (signals A, B, C ) 0-solvated by the acid molecules (signal D) were discussed earlier in this paper. The O H N protons are probably represented by the A’ signal (Figure 6) which is recorded together with B’ and C’ signals, and its positions are typical for relatively weak OHN comple~es!~ The acidic signals and formyl signals are partially overlapped, but it is clear that the A’ intensity exceeds the B’ one and that this difference vanishes when R increases. The acidic signals D:A:A’ in the 1.7 I R I 2.0 range (Figure 6) form broad triplets with 1 :n:l intensities, where n > 2 and n = 2 for R < 2 and R = 2.0, respectively. This indicates that at -70 O C a-complexes interact with the acid molecules through the nitrogen (signal A’) and the SO3 group (signal D) with approximately equal probability. According to this idea, the contributions of different kinds of complexes to D, A, A’, B, and B’ signals can be expected (Table IIJ), and the observed intensities of the signals are proportional to the corresponding concentrations:
< I B >> Ig’ (C, + Cy) < (C, + C,) >> Cf
+
+ C,,):(C, + Cy):Cy, cyt:ca:c7r = 1:1:1
It seems that the following associates exist in the solution for R = 3 (AH)*-AH-O(DMF)N AH-AH-O(DMF)N
CH3
(DMF)’
