M. S.Sambhi and S.K. Khoo
666
The Nature of Bonding in Amine-Iodine Complexes Manjit S. Sambhi* and S. K. Khoo Department of Chemistry, University of Malaya, Kuaia Lumpur, Malaysia (ReceivedJune 2 1, 1974; Revised Manuscript Received December 3, 1974) Publication costs assisted by the Departmentof Chemistry, University of Malaya
The enthalpy of formation of molecular complexes arises from charge transfer and other forces, including coulombic, induction, dispersion, and exchange repulsion forces. The use of a theoretical expression which equates enthalpy of formation with energy terms arising from charge transfer and other forces can provide information about the magnitudes of these energy terms, and thus provide an insight to the nature of bonding in amine-iodine molecular complexes.
Introduction The extent to which charge transfer and other forces contributes to the observed ground-state properties of molecular complexes is a topic of considerable interest.1-4 A relationship5 derived from AHo and the enhancement of dipole moment for amine-iodine complexes shows that the large observed -AHovalues for these complexes is a consequence of strong charge-transfer interactions since the energy contribution made by other forces to AHo has essentially a constant positive value of 10.4 kcal mol-l. We have confirmed6 this result by the use of a theoretical equation of Person7 and we now proceed to determine the magnitude of the contribution made by non-charge-transfer forces to AHo of amine-iodine complexes. Theory a n d Results The ground and excited states of a complex (D,A) derived from the interaction of an electron donor (D) and an electron acceptor (A) can be described8 by the wave functions ~ J N=
a$o(D, A)
+ b+i(D'-A')
and = a*$,(D'-A-)
- b*$o(D, A)
The terms $0 and +1 refer to the no-bond and dative-bond wave functions, respectively. The ground-state energy ( E N ) of a complex can be written as
EN = RN -I-Eo = AH" where RN is the resonance energy due to the charge-transfer force in the complex, while Eo is the energy change due to other interactions when the donor and the acceptor come together. The coulombic, induction, and dispersion forces are attractive, while the exchange repulsion works against them, thus making Eo either negative, zero, or positive. For molecular complexes which can be satisfactorily described by one charge-transfer state and two resonance structures $0 and $1, RN is given by the theoretical expression
-
+
The term (El Eo) = ID - EA Ec - Eo where EA is the electron affinity of the acceptor and Ec is mainly the coulombic attractive energy associated with the dative structure of the complex. If ( E A - Ec EO)is designated by C1, then AH" is given by the expression +I.
+
AW = {ID - c l - 2 & s - [(ID - c,)' 4P,S(ID - C,)
+ 4p02]i'2}/2(1 - S2) + E ,
Equation 2 can be conveniently expressed as AW=Z-tE, (3 ) where 2 represents the first term on the right-hand side of eq 2. For a series of complexes derived from structurally related donors with a common acceptor, it has been shown9 that the parameters PO, C1, and S can be assumed to be fairly constant, but the empirical procedure usedg to obtain these parameters can only provide a range of reasonable rather than precise values. Hence, direct substitution of the values of these parameters obtained empirically can lead to erroneous estimations of RN and Eo if the values used turn out to be significantly different from the true values of the complexes. For amine-iodine complexes i t was foundlo that a Hammett plot of AHo against u* is linear for complexes which are relatively free of steric factors and we will assume Po, C1, and S as constant for these complexes. Table I shows the ID and AHo values for these complexes. It was empirically found by the use of a least-squares fit program that a linear relationship with a correlation coefficient of 0.99 exists between AHo and 2 values calculated for values of 00, C1, and S of -1 to -4 eV, 6 to 9 eV, and 0.15 to 0.55, respectively, at parameter intervals of 0.05. This result can be formalized by the expression AH" = mZ
- ~ P , s ( E, E,)
, ) 2
+ 4fi021i/2)/2(1
-~
2 )
(1)
where E1 is the energy of the dative structure, Po is a resonance integral, and S is the overlap integral between $0 and The Journal of Physical Chemistry, Vol. 79, No. 6, 1975
+k
(4 )
where
m
=f(Po, C,, s)
and k =f(Po,
[(E*- ~
(2)
c,, S )
Thus, when m = 1, eq 4 becomes identical with eq 3 and h = Eo. Table I1 shows the solutions obtained for Eo together with the appropriate values of Po, C1,and S. The recommended range of valuesg for 00,C1, and S are
Communications to the
667
Editor
TABLE I: AHoand ID Data for Amine-Iodine Complexesn Donor
-AH",eV
ZD,eV
Methylamine Et hylamine n - Butylam ine Dimethylamine Trimethylamine Reference 10.
0.308 0.321 0.364 0.425 0.525
8.97 8.86 8.71 8.24 7.82
solutions are markedly different from the recommend values of S,while solutions 2,5, 6, and 7 may be reasonable since two out of three parameters Po, C1, and S associated with these solutions have values which are within the recommended range of values.
TABLE 11: Solutions of Eo with Appropriate Values of PO, C1, and S Solution 1 2 3 4 5 6
7
-Po,
Ci
k = E 0,
f
eV
eV
S
??2
1.500 1.500 2.000 2.000 2.500 2.500 3.000
6.500 7.000 6.000 6.500 6.000 6.500 6.000
0.200 0.450 0.250 0.400 0.400 0.550 0.500
0.990 1.018 1.006 0.990 0.999 0.996 0.983
eV 0.298 0.293 0.536 0.520 0.756 0.737 0.966
Conclusion A linear relationship can be found between AHoand RN for these amine-iodine complexes which suggests that Eo is essentially a constant. It has a large positive value of 11.99 kcal mol-' which is in good agreement with the value of 10.4 kcal mol-l obtained from the use of a relationship between AHo and the enhancement of dipole moment.5 The large positive value indicates that repulsive forces dominate the term Eo and that charge-transfer forces account for the large observed -AH" values for these complexes. The RN contribution to AHo of these complexes varies from -19.1 to -24.1 kcal mol-l. References and Notes
-1.7 to -3 eV, 6.5 to 7.5 eV, and 0.4 to 0.5, respectively, thus solution 4 provides the best values of Eo of 0.520 eV mol-' (11.99 kcal mol-l) as values of Po, C1, and S1 associated with this solution have values which fall within the recommended range of values. Solutions 1 and 3 can be rejected on the grounds that S values associated with these
(1) M. W. Hanna, J. Amer. Chem. Soc., 90, 285 (1968). (2) J. L. Lippert, M. W. Hanna, and P. J. Trotter, J. Amer. Chem. SOC.,91, 4035 (1969). (3) R. J. W. Le Fevre, D. V. Radford, and P. J. Stiles, J. Chem. SOC.5, 1297 (1968). (4) R. S. Mulliken and W. 6.Person, J. Amer. Chem. Soc., 91, 3409 (1969). (5) H. Ratajczak and W. J. Orville-Thomas, J. Mol. Structure, 14, 149 (1972). (6) M. S. Sambhi, J. Mol. Structure, submitted for publication. (7) W. 6.Person, J. Chem. Pbys., 38, 109 (1963). (8) R. S.Mulliken, J. Amer. Chem. SOC.,74, 811 (1952). (9) R. S. Mulliken and W. 6.Person, "Molecular Complexes: A Lecture and Reprint Volume," Wiley, New York, N.Y., 1969. (IO) H. Yada, J. Tanaka, and S. Nagakura, Bull. Chem. SOC.Jap., 33, 1660 (1960).
COMMUNICATIONS TO THE EDITOR
Some Unmeasured Chlorine Atom Reaction Rates Important for Stratospheric Modeling of Chlorine Atom Catalyzed Removal of Ozone Publication costs assisted by the Division of Research, U.S. Atomic Energy Commission
C1 atoms can then be released again through the attack of OH and HC1 as in (4). From the measured concentrations OH
C1
Sir: Chlorine atoms when injected into the stratosphere at 25 km or above can initiate a chain reaction with odd oxygen involving principally reactions 1-3.1-9 The primary c1
+
c10 C10
+
o3 +
0
NO
--
c10
+
+
0 2
(1)
c1 + o2
(2)
C1 iNO,
(3)
manner in which this C10, chain is interrupted in the stratosphere is through the reaction of C1 with various hydrogen-containing species with the formation of HC1. The
+
HC1
+
H,O
+
C1
(4)
of CHd1O and the reaction rate coefficients,ll it is clear that reaction 5 with CH4 is quite important in the stratosphere,
+
CH,
--+
HC1
+
CH,
(5)
and it has been regularly assumed to be the chief HC1 forming reaction between about 30-50 km. Our purpose here, however, is to estimate the possible contributions from other hydrogen-containing species in the stratosphere, with the primary intention of discovering whether CH4 is an overwhelmingly dominant H-abstraction source. Several recent calculations indicate that C10, -catalyzed removal of odd oxygen is becoming an increasingly serious global environmental problem through the release of C1 atoms in the stratospheric photodissociation of CF2C12 and CFC13.1-3, 7-9 The Journal of Physical Chemistry, Vol. 79, No. 6, 1975
