2637
J. Phys. Chem. 1981, 85,2637-2639
Inversion of Close-Lying ' n r * and ' m *States in 2-Amlnopyridine by Protonation. A CNDO Study A. C. Testa" Department of Chemlstty, St. John's Unlverslty, Jamaica, New York 11439
and U. P. Wlld" Laboratorlum fur Physlkallsche Chemie, Eidenosslsche Technische Hochschule, Zurich 8092, Switzerland (Received: October IO, 1980; In Final Form: May 3 1, 198 1)
A CNDO study of the protonation of 2-aminopyridine is shown to correctly predict the inversion of the two lowest excited singlet states and the increased fluorescence that accompanies protonation. A potential energy minimum is observed when the proton is 1.03A from the ring nitrogen, and a significant decrease in the coefficient of the nN orbital of pyridine nitrogen occurs upon protonation. The oscillator strength of the first h a * state appears to correlate with the fluorescence efficiency, which supports the existence of close-lying h a * and h a * singlets for the neutral molecule in a nonpolar solvent.
Introduction The fluorescence behavior of aromatic molecules containing nonbonding electron pairs are generally expected to show sensitivity to solvent polarity and hydrogen bonding. In particular we have previously reported that the fluorescenceof 2-aminopyridine increases dramatically from 4F = 0.040 in cyclohexane to 4 F = 0.64 in 0.1 N H2S04,1v2which is attributed to removal of the nonbonding electron pair upon protonation. In view of the widespread use of MO calculations it appeared attractive to consider a CNDO study of the protonation of 2-aminopyridine, with particular interest in testing its ability to predict molecular luminescence. Noteworthy is that protonation adds no additional electrons to the MO basis set. Specifically we have investigated the variation of the proton distance from the ring nitrogen on orbital energies. Semiempirical and ab initio studies of hydrogen bonding and proton affinities have been r e p ~ r t e d .Among ~~ some of the questions we hoped to answer were the following: (1)how do the n r * and r?r*levels move and is there an inversion of states as we protonate the ring nitrogen; (2) do the predictions correlate with the marked increase in fluorescence behavior upon protonation; and (3) can oscillator strengths be used to predict variation in fluorescence? With the aim of trying to correlate CNDO calculations with molecular fluorescence the present study was undertaken. Experimental Section Materials. 2-Aminopyridine was purified by vacuum sublimation at 5 X lo4 torr to yield colorless crystals. Fluorimetric grade cyclohexane and sulfuric acid were used as received, after verifying that they contributed no impurity fluorescence. Absorption and fluorescence measurements were made in l-cm cells. MO Calculations. Prior to the CNDUV99 calculations,' the molecular geometry was optimized by a force field calculation, QCFF/PI? In the CI procedure all excitations below 10 eV were used, which provided 25 singly excited (1) S. Babiak and A. C. Testa, J. Phys. Chem., 80, 1882 (1976). (2) R. Rusakowicz and A. C. Testa, J.Phys. Chem., 72, 2680 (1968). (3) P. Schuster, Theor. Chim. Acta, 19, 212 (1970). (4) J. Catalh, M. Yafiez, and J. I. Fernandez-Alonso, J. Am. Chem. Soc., 100,6917 (1978). (5) J. Catalh, 0. Mb, P. Perez, and M. YZez, J.Am. Chem. SOC.,101, 6520 (1979). (6) J. E. Del Bene, J. Am. Chem. SOC.,101, 7146 (1979). (7) H. Baumann, QCPE, 11, 333 (1977). (8)A. Warshel and M. Levitt, QCPE, 11, 247 (1974).
TABLE I: Bond Distances and Angles Used in t h e the M O Calculations 710
bond
1-2 1-6 2-3 2-7 3-4 4-5 5-6
length, length, A bond A atoms angle
1.379 7-12 1.006 1.370 7-13 1.006 1.427 3-11 1.082 1.418 4-10 1.083 1.400 5-9 1.083 1.410 6-8 1.085 1.403 (1-14variable)
1-2-3 1-2-7 2-3-4 3-4-5 4-5-6 5-6-1 6-1-2
119.0 120.4 119.8 119.5 119.6 120.3 121.8
atoms angle
2-7-13 2-7-12 1-6-8 4-3-11 4-5-9
121.0 121.1 120.1 119.9 120.1
configurations in arriving at the final energy levels. Two center two-electron repulsion integrals were evaluated according to the Mataga-Nishimoto formula. A minimum in the potential energy curve was sought by varying the proton distance along a path colinear with the plane of the ring and the ring nitrogen. A summary of the bond distances and angles used are given in Table I.
Results The variation of the proton distance from the ring nitrogen resulted in the potential energy curve which is presented in Figure 1. It is clearly seen that a minimum arises at 1.03 A, which is in reasonable agreement with known N-H bond distances. The molecular state diagram for the 2-aminopyridinium ion, generated from the CNDUV99 calculations, is given in Figure 2, together with the result for 2-aminopyridine. The value in parentheses is the predicted oscillator strength for the lmr* transition, while the h a * states appear as dashed lines. The oscillator strength for the h?r*transitions are not given, since in the CNDO procedure they have zero transition moments. Two features which deserve attention in Figure 2 are (1) the h a * state which is S1 in 2-aminopyridine becomes S3in 2-aminopyridinium upon protonation, and (2) the lowest excited singlet state, S1, which is I?r?r* in the latter, has undergone a fourfold increase in oscillator strength, relative to the neutral molecular where it is Sp.
0022-3654/81/2085-2637$01.25/0@ 1981 American Chemical Society
2638
The Journal of Physical Chemistry, Vol. 85, No.
18, 1981
-285.
TABLE 11: Summary of Singlet Energies (CNDUV99) enerev. eV confieuration 2-Aminopyridine ( n = ~0.701) S,(nn*) 3.80 16-19 SAan*) 4.52 18-1 9 17-20 S,(nn * 1 5.48 16-20 S,(nn*) 5.62 18-20 17-19 18-19 2-Aminopyridinium ( n = ~0.208) S,(nn*) 4.17 18-19 17-20 17-19 18-20 16-19. 16-20 14-19 14-21
2-AMINOPYRIDINIUM
I.o
1.2
1.1
I.3
. A.
RN."*
Testa and Wild
1.4 UV ABSORPTION OF
Flgure 1. Plot of CNDUV99 energy as a function of the distance between the proton and the ring nitrogen in 2-aminopyridine. 5c
r
/ 46
',
I-------.......;0~04 .......
s3 - - - - - - - 42
i
I
~
38 (0.043) 1O3d
in OJM (OH-) ( 3 1 6 . 8 x I O - ~ M in O.IM (H')
(21 6.5 x l d 5 M
s3
T5 s2
I
............................. 6
T
75
5,
2-AMiNOPYRIDINE
( I ) 5 . 4 x I J - ~ M in CH
'.
T4
/
34
WAVELENGTH, nm,
Flgure 3. UV absorption spectrum of 2-aminopyridlne in nonpolar, basic and acidic media. 30 FLUORESCENCE
OF
2-AMP
AND
2-AMPH'
T3 ......................... T2
-
26
>
t W z
22
e
I
W V
18
W
Figure 2. Molecular electronic state diagram for 2-aminopyridine and rep2-aminopyridinium obtained from CNDUV99 calculations. (resents 'na' and - represents 'aa*.)The 'na* states represented as dotted lines were estimated by assuming that they lie 2500 cm-I below 'na*. The value in parentheses is the CNDUV99 derived oscillator strength.
---
In Figure 2 an estimate of 2500 cm-l was used for the singlet-triplet splitting of na* transitions. This value was deduced from a low-temperatureflash spectroscopic study of aminopyridines? The prediction by Ridley and ZernerlO that a 3na* (3B1)state lies at 33 223 cm-' in pyridine adds further support for the separation energy used in the present study. ~
~~
~
(9) J. Wolleben and A. C. Testa, J. Photochem., 8, 125 (1978). (10)J. R. Ridley and M. C. Zerner, Theor. Chirn. Acta, 42,223 (1973).
In (I W
3
20
25
30
5,
cm:'
35
x
Figure 4. Fluorescence spectrum of 2-aminopyrldine In (1) cyclohexane, (2) ethyl alcohol, and (3)0.1 M HCI (285-nm excitation).
A summary of the singlet-state energies for 2-aminopyridine and 2-aminopyridinium, together with their configurational make-up, is given in Table 11. The absorption and fluorescence spectrum of 2-aminopyridine and its protonated form are given in Figures 3
2839
J. Phys. Chem. 1981, 85,2639-2642
and 4, respectively, where it is evident that the energy of the protonated form is lower in both the absorption and fluorescence spectra. The lowest singlet state which is h a * in a nonpolar solvent like cyclohexane is not discernible in the absorption spectrum due to the larger extinction coefficient of the lowest mr* transition, which, strongly mixed with the lnA* transition, dominates the first absorption band; consequently, the absorption and fluorescence spectra correlate with the lowest l m * states in both the neutral molecules and the protonated form as predicted in Figure 2.
TABLE 111: Comparison of Fluorescence Data with CNDUV99 Predictions for 2-Aminopyridine CNDUV99 fluorescencea n orbital energy, 'ZN TF> eV coeff f ( n n * ) @ f ns S, 2-AMP -11.16 0.701 0.043 0.04 24.2 nn*
Discussion I t is evident from the results that CNDUV99 calculations correctly predict (a) the ordering and movement of singlet states that account for the fluorescence behavior of 2-aminopyridine, and (b) that the oscillator strength of the lowest l m * transition is a measure of fluorescence tendency when 'nn* and lmr* are close lying as they are for 2-aminopyridine in nonpolar hydrocarbon so1vents.l Although the satisfactory correlation between the oscillator strength of the lowest m* singlet and the fluorescence yield may be considered simplistic, this procedure appears to be useful for some aromatic heterocyclic molecules. Our recent satisfactory correlation of the oscillator strength of the lowest lmr* transition with the fluorescence yield for five methyl-substituted 2-aminopyridine molecules gives a reasonable degree of confidence that the correlation we find in this study is meaningful.'l Although the energy spacing between lnr* and l m * in the neutral molecule is predicted by CNDUV99 to be 6000 cm-', the absorption spectrum indicates that the actual splitting is much smaller and involves vibronic
2-AMP
-
(11) A. C. Testa and U. P.Wild, J. Phys. Chem., 83, 3044 (1979).
-
~
in CH
2-AMP in EtOH
0.18
5.1 nn*
2-AMP in 0.1 M OH-
0.03b
7.2 nn*
in 0.1 M H+
-17.31
0.208 0.172 0.64
11.2 nn*
a Data from ref 1. Low fluorescence yield due to hydroxide ion quenching, i.e., RNH, *' t OH- + RNH- +
H,O.
coupling. Clearly it is the inversion of singlet states upon protonation, which is properly predicted and of fundamental importance in understanding the fluorescence behavior of 2-aminopyridine. A comparison of the CNDO calculations with the fluorescence data for 2-aminopyridine is presented in Table I11 where it is seen that the low fluorescence yield in a nonpolar hydrocarbon solvent can readily be understood in terms of a lowest lnr* singlet, which is vibronically coupled to the l m * singlet with an oscillator strength of 0.043, while in the protonated form the lowest singlet is m* and the oscillator strength has been increased to 0.172. Thus increased fluorescence upon protonation arises from a shift of the n orbital on the ring nitrogen to higher energies. Further the increased oscillator strength of the lowest m* singlet state predicted by CNDUV99 is expected to manifest itself as increased fluorescence upon protonation as is observed experimentally.
Quantum-Chemical Studies of the Interaction between Hydrogen Fluoride and Cyanide Roger L. DeKock" and Debra S. Caswell Department of Chemisw, Calvln College, Grand RapMs. Mlchigan 49506 (Received: January 26, 198 1; In Flnal Form: June 1, 198 1)
The electronic structure and the geometry of the strong hydrogen-bonded ion FH.-.CN- have been studied by using the semiempirical MNDO and ab initio molecular orbital methods. The calculations indicate the linear ion to form without a potential energy barrier by reaction of either HF with CN- or P with HCN. At the 4-31G basis set level, the H-F bond length is increased by 0.07 A whereas the C-N- bond length remains unchanged upon formation of FH...CN- The hydrogen-bond strength is calculated to be (MNDO) 17 kcal/mol, (STO-3G) 27 kcal/mol, (4-31G) 36 kcal/mol, and (6-31G**) 28 kcal/mol. These results are typical in that MNDO underestimates whereas 4-31G overestimates hydrogen-bond energies. The 431G optimized Ha. -C bond distance is 1.64 A. The calculated geometry for FH...CN- by the MNDO method is very close to that obtained at the 4-31G ab initio level. However, for the FH...NC- isomer the MNDO results indicate a much longer and weaker bond than do the 4-31G ab initio results. The FH...NC- isomer is found to be only 1.1 kcal/mol less stable than FH...CN- at the 4-31G level. Hence, the results presented here cannot definitively pinpoint which of the two isomers has been observed recently in a matrix isolation study.
.
Introduction In a recent study, Aultl reported matrix isolation infrared studies of the M+FHCN- ion pairs where M+ is an alkali metal cation. These studies indicated that the anion is a strongly bound "type I" hydrogen-bonded system. (1) Ault, B.S.J . Phys. Chem. 1979,83, 2634. 0022-365418 112085-2639$01.25/0
Type I refers to unsymmetrical hydrogen bond. Ault also quotes J. Beauchamp as bracketing the H-bond energy between 13 and 45 kcal/mol from gas-phase ion cyclotron experiments* From the point of view of the present quantum-chemical study, there are two interesting aspects of the formation of the FHCN- ion that deserve further investigation. First, Ault observed the formation of the same ion upon reaction 0 1981 American Chemical Society
