J. Phys. Chem. 1991, 95, 8520-8524
8520
directly involved in the electronic transition. In the twisted conformation, the lowest energy transition is from a highest occupied molecular orbital (HOMO), which is purely localized on the donor group NMe2, to the lowest unoccupied molecular orbital (LUMO), localed on the benzonitrile m ~ i e t y . ’ ~Therefore, .~ the HOMO to LUMO transition results in extensive charge transfer. The MO diagram for 9-amino-10-cyanoanthracene is shown in Figure 3. It can be seen that the transition from the HOMO to the LUMO is localized mainly on the anthracene ring. Consequently, there is no significant change in the electron density at the nitrogen atom of the NH2 group upon electronic excitation, thus reducing the absolute value of Ap. The MO diagrams for compounds 4-5 were similar in nature to that for compound 6. Anthracene derivative with only an acceptor group, such as 9-
nitroanthracene, showed a similar behavior for the Ap value.17 Recent reports by Rettig18 and othersI9 have pointed to the validity of the minimum overlap rule in describing the chargetransfer properties of aromatic molecules having donoracceptor groups. The results described in this paper also confirm the observations made earlier for other compounds exhibiting TICT behavior.
Acknowledgment. We are grateful to the Natural Sciences and Engineering Research Council (NSERC) of Canada for financial support for this work. Registry No. 4, 133826-03-6; 5, 135614-62-9; 6, 14789-44-7. (17) Sinha, H. K.; Yates, K. J . Chem. Phys. 1390, 93,7085. (18) Rettig, W. Angew. Chem., In?. Ed. Engl. 1986, 25, 971. (19) BonaEiE-Kouteck5, V.; Michl, J. J. Am. Chem. Soc. 1985,107, 1765.
(16) Ertl, P. Collect. Czech. Chem. Commun. 1990, 55, 1891
n = 0-59 and X = OH, 0, 02,
Chemistry of Large Hydrated Anion Clusters X’(H,O),, and 03. 2. Reaction of CH3CN X. Yang: X. Zbang, and A. W . Castleman, Jr.*
Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received: February 26, 1991; In Final Form: June 5, 1991)
The kinetics and mechanisms of the gas-phase reactions of acetonitrile with large hydrated anion clusters X-(Hz0),,+s9, X = OH, 0, 02,and 03, were studied in a fast flow reactor under thermal conditions. OH-(H20)n.+1 react with CH3CN at near collision rate via proton-transfer and ligand-switching mechanisms; further hydration greatly reduces the reactivity of OH-(H20)n>ldue to the thermodynamic instability of the products compared to the reactants. On the contrary, for all cluster sizes studied, O-(H20), reacts with CH3CN at near the collision rate via a hydrogen transfer from acetonitrile to the anion clusters. A new reaction channel was found for the reaction of 0-with CH3CN to form CHCN-, which can react further with CH’CN to form CHICN and CH2CN-. Only very slow associations were observed for the reactions of 0,and 0; and their hydrates. The possible application of the present experimental results to an understanding of the hydrolysis mechanism of CH3CN in aqueous solution in the presence of OH- and protons as a catalyst is also discussed.
Introduction Most chemical reactions occur in solution, and a vast number of solution reactions are of an ionic nature. However, a molecular-level understanding of solution chemistry is still lacking for many systems due to extreme difficulties in both experiment and theory to handle the solvent effect and the multiparticle interactions present in the condensed phase.’ Indeed, the intrinsic properties of ions or molecules can be more readily obtained in the gas phase since no interference of a bulk solvent exists there; a promising avenue of research employs clusters in order to bridge an understanding of similarities and differences in properties and reactivity between the gas phase and the condensed phase. It has been found that the effect of solvation can alter not only the molecular properties but also the reaction mechanism. For example, the well-known reaction of acetonitrile in acidic (or basic) aqueous solution can proceed via a proton (or OH-) catalyzed hydrolysis reaction mechanism? CH3CN + 2 H 2 0 -% CH’COOH
+ NH3
(1)
However, in the gas phase,’ CH3CN reacts with H30+ by a proton-transfer reaction mechanism: CH’CN
+ H30+
-
(CH3CN)H+ + H 2 0
(2)
Interestingly, adding one water onto H30+ as a ligand will change the proton transfer to a ligand-switching mechanism: ‘Present address: Exxon Research and Engineering Company, Clinton Township, Route 22 East, Annandale, NJ 08801-0998.
0022-3654191/2095-8520$02.50/0
-
CH’CN + H30+(H20) (CH3CN)(H20)H++ H 2 0 (3) Since the reactivities of a large number of ion-molecule reactions are now well established in the gas phase? it is a natural step to study the effect of solvation on simple ion-molecule reactions by clustering a variety of different sizes of solvent molecules onto the ions.s It has been recognized that as the number of the solvent molecules gets large, the gas phase large cluster ionmolecule reactions will begin to show some behavior analogous to those operational in solutions.6 In the past year, for the first time large protonated water clusters H+(H20)n=1-603.7.8 and negative cluster ions X-(H20)nIM9 (X = OH, 0, 02,and O3)+I1were produced and studied under well~
~
~
~~~
~
( I ) For example: Moore, J. W.; Pearson, R. G . Kinetics and Mechanism, 3rd ed.;Wiley: New York, 1981. (2) For example: Daniels, F.; Williams, J. W.; Bender, P.; Alberty, R. A.; Cornwell, C. D.; Harriman, J. E. Experimental Physical Chemistry, 7th ed.; McGraw-Hill: New York, 1970; p 139. (3) Yang, X.; Zhang, X.; Castleman, A. W., Jr. Kinetics and Mechanism Studies of Large Protonated Water Clusters H+(H20), n = 1-60, at Thermal Energy. In?. 5. Mass Spectrom. Ion Processes, ih press. (4) Ikezoe. Y.; Matsuoka, S.; Takebe, M.; Viggiano. A. Gas Phase IonMolecule Reaction Rate Constants through 1986; Maruzen Company: Tokyo, 1987. (5) Keesee, R. G.; Castleman, A. W. In Ion and Cluster Ion Spectroscopy and Structure; Elsevier: New York, 1989; pp 275-327. (6) Castleman, A. W., Jr.; Keeset, R. G . Chem. Rev. 1986, 86, 589. Castleman, A. W., Jr.; Keesee, R. G . Acc. Chem. Res. 1986. 19, 413. Castleman, A. W., Jr.; Kcesec, R. G . Science 1988, 241, 36. (7) Yang. X.; Castleman, A. W., Jr. J . Am. Chem. Soc. 1989, I l l , 6845. (8) Yang, X.; Castleman, A. W., Jr. Temperature and Cluster Size Dependence Studies of Reactions of Protonated Water Clusters with Acetonitrile. J . Chem. Phys. 1991, 95, 130.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 8521
Chemistry of Large Hydrated Anion Clusters defined conditions of temperature and pressure. We observed at low temperature that3 for the reaction of large protonated water clusters with CH3CN, the reaction mechanism ultimately changes from a switching reaction: CH$N
+ (H2O),H+
-w
(CH,CN)(H20),lH+
+ H20
TABLE I:' Rate Constants for the Reactions of OH-(HlO), with CH,CN at T = 300 K reported
(4)
to an association mechanism: CH3CN
+ (HzO),H+
+
(CH3CN)(H2O),H+
n
this work
ref 18
ref 17
0 1 2
4.3 3.8 0.12
3.3 3.0 0.14
4.4
theorym 5.3 4.2 3.8
Units: 1 O+ cm3/s.
(5)
as the cluster size n is increased to larger than about 15. In the present work the molecule CH3CN is chosen for studying reactions with large anion clusters not only because the results will be complementary to those of the large protonated water clusters3J but also for some interesting applied reasons as well. For example, the hydrolysis of CH3CN, catalyzed by acids or bases, is still one of the methods for the synthesis of CH3COOH.12 Of similar importance, CH$N has been found to play a key important role in the ion chemistry of the stratosphere and the troposphere.I3 Laboratory3J4and in situ measurement~l~ have established that H+(CH3CN),,,(H20), clusters are the major positive ions in the lower stratosphere. It has been suggestedI6 that CH3CN also may be involved in the evolution of negative ions in the stratosphere. Although several research groups have studied the reactions of protonated water clusters with CH3CN,SJ4J7the only previous work for the reaction of hydrated anion clusters with CH3CN was reported by Ebhme and co-workers on OH-,I7 and Paulson and co-workers on OH-(H20)n+2,180-, and OFi6 at T = 240-363 K. Herein, we present experimental results of the rate constant measurements and determination of the mechanisms of reactions of CH3CN with X-(HzO),,+s9, X = OH, 0,O,,and O3at both room temperature and T = 130 K. A brief discussion of the hydrolysis reaction mechanism of acetonitrile in aqueous solution in terms of gas-phase ion-molecule reactions and the solvation effect is also given.
TABLE 11:" Rate Constants for the Reactions of O-(H20), with CH&N at T = 300 K
Experimental Section The experiments were performed using a fast flow reaction apparatus affixed with a high-pressure ion source and operated at both room and low temperature. The instrument has been described in detail in earlier publication^.^.'^ Briefly, OH-(H,O), and O-(H20), anion clusters are formed in the ion source by discharge ionization of a water/helium mixture. Adding a small amount of 0,into the source greatly enhances the intensity of 02-(H20), and 03-(H20), ions. Thereafter, the clusters are carried by He from the ion source into the flow tube where they react with neutral reactants, Le., CH3CN in this case. The reactant neutral is diluted in He and added into the flow tube through a heatable reactant gas inlete3 Most of the gas in the flow tube is pumped away by a roots pump, and only a small portion is sampled into the detection chamber where the reaction
at two different temperatures. At room temperature, only clusters OH-(H20)n=&3?0-(H20)n=&3, 01(H20)n=+2 and O
