2820
J . Phys. Chem. 1990, 94, 2820-2828
ethylenediamine can also be interpreted in terms of Rydberg states. However, in this latter case, the strong molecular dipole moment should induce an asymmetry in the Rydberg orbitals and then favor the appearance of nonlinear optical properties. Such studies are under progress.
Acknowledgment. L. Facchini, F. Robert, G. Collin, M. Maitrot, J. P. Parnex, and C. Legrand are thanked for their experimental help. Registry No. N-Oxide of triethylenediamine, 18503-52- I .
Modified Oxidation Number As Applied to Organic Compounds Containing Nitrogen Atoms. Electron Number Analysis with ab Initio Molecular Orbital Wave Functions Keiko Takano, Reiko Yoshimura, Mari Okamoto, and Haruo Hosoya* Department of Chemistry, Ochanomizu University, Bunkyo- ku, Tokyo 112, Japan (Receioed: July 12, 1989; In Final Form: October 20, 1989)
Accurate electron number analysis around the specified atoms in several series of nitrogen compounds containing C, H, 0, and N atoms was performed with ab initio Gaussian-type molecular orbital wave functions. The oxidation number of the hydrogen atom in NH bonds is assigned +3/4, which is just between OH+'and CH+Il2already assigned. For those saturated and unsaturated nitrogen-containing organic compounds in which large electronic migration is not expected, a set of simple additivity rules were derived for the group (modified) oxidation numbers. For those unsaturated compounds containing an amino group conjugated with a double bond the extent of the electronic migration was measured qualitatively from the contour maps of u- and *-electron distributions.
Introduction
We have been analyzing the quantum chemical aspects of the oxidation number with ab initio molecular orbital wave funct i o n ~ . ' - ~In our previous papers it was shown that the difference spherically averaged electron density, Apo(R), which can be calculated analytically with Gaussian-type wave functions,'S6 has a clear proportional relationship with the classically assigned oxidation number of the respective atoms in inorganic molecules such as chlorine and sulfur compounds, although the actual amount of deformation of the electron density is as small as one-tenth of the oxidation number. On the other hand, in organic chemistry the concept of the oxidation number has rarely been applied contrary to the fact that great importance is realized in the oxidation-reduction reactions. However, our analysis shows that around the carbon atoms in organic molecules containing C, H, and 0 atoms a clear stepwise change was observed similar to the case of inorganic compounds. The absolute values of each Apo(R)change in this case is a little smaller than that of S and CI atoms, revealing smaller electron density changes between atoms of similar electronegativity, such as between C and H. After a slight modification to the assignment of the oxidation number, e.g., + I /2 for the H atom in CH bonds in contrast to the formal value of + I for the H atom in OH bonds, we could obtain a systematic understanding of the oxidation states of the component atoms in organic molecules as well as in inorganic molecules. For example, the C atoms in CH4, C H 3 0 H , HCHO, H C 0 2 H , and C 0 2 are given -2, - l / 2 , + I , +5/2, and +4, respectively, as the modified oxidation numbers instead of ( I ) Takano, K: Hosoya, H.; Iwata, S. J. Am. Chem. SOC.1982, 104,
3998-4005. (2) Takano, K.; Hosoya, H.; Iwata, S. J. Am. Chem. SOC.1984, 106, 2787-2792. (3) Takano, K; Hosoya. H.; Iwata, S. In Applied Quantum Chemistry; Smith, V. H., Schaefer I l l , H . F., Morokuma, K., Eds.; Reidel: Dordrecht, 1986; pp 375-393. (4) Takano, K . ; Hosoya, H.: Iwata, S. J. Chem. SOC.Jpn. 1986, I I . 1395-1404. (5) Takano, K.; Okamoto, M.; Hosoya, H. J. Phys. Chem. 1988, 92, 4869-48 7 5 . (6) Iwata, S. Chem. Phys. Lett. 1980, 69, 305-312.
0022-3654/90/2094-2820$02.50/0
-4, -2,0, +2, and +4. The simple additivity rules for the modified oxidation numbers were also obtained for hydrocarbons and their oxides, in which large electronic migration and steric hindrance are not expected. The overall change of the formal oxidation number of nitrogen is known to be as large as eight, from -3 of NH3 to +5 of HNO,. However, in our preliminary study on two series of compounds, NH3, N2H,, N2H2,and N2 and HNO, H N 0 2 , and H N 0 3 , the Apo(R) value around the N atom is actually changed stepwise, but its overall change from N H 3 to H N 0 3 is considerably smaller than those for the case of S and C1 compound^.^ Nitrogen compounds have been the target of theoretical studies of a variety of electronic features, such as proton affinity,'$* oxidation state,9 geometries,I0-l3 and reaction^.^^^'^ In this paper our electron number analysis was focused on the elucidation of the oxidation state of the N atom in organic compounds containing functional groups such as NO,, NH,, CN, and -N=. From the systematic electron number analysis thus performed, the oxidation number of the hydrogen atom in N H bonds can be assigned +3/4, which is just the mean value of OH+' and CH+'12hydrogen atoms, which reminds us of the fact that the N atom lies just between the C and 0 atoms in the Pauling electronegativity scale as C(2.5), N(3.0), and O(3.5). Further, the simple additivity rules for group oxidation numbers, which was found in the previous study, can be extended to both the saturated and unsaturated nitrogen compounds. For several unsaturated compounds, in which large a-electronic migration ( 7 ) M6, 0.;de Paz. J. L. G.; YBnCz, M. J . Phys. Chem. 1987, 91. 6484-6490. ( 8 ) Botschwina, P. Chem. Phys. Lett. 1987, 139, 255-261. (9) Baird, N . C.; Taylor, K. F. Chem. Phys. Lett. 1981, 80, 83-86. (IO) Benioff, P.; Das, G.;Wahl, A. C. J. Chem. Phys. 1976, 64, 71C-717. ( I I ) Bock, C.; Trachtman, M.; Schmiedekamp, A,; George, P.; Chin, T. S. J . Comput. Chem. 1983, 4, 379-389. ( I 2) Yamashita, K.; Morokuma, K. Chem. Phys. Lett. 1986, 131,237-242. ( I 3) Sedano, E.; Sarasola, C.; Ugalde, J. M.; Irazabalbeitia, I . X.; Guerrero, A. G. J . Phys. Chem. 1988, 92, 5094-5096. (14) Marudarajan, V.; Segal, G. A. Chem. Phys. Lett. 1986, 128, 1-4. ( I 5 ) (a) McKee, M. L. J . Phys. Chem. 1986, 90,2335-2340. (b) McKee, M . L. J Am. Chem. SOC.1986. 108. 5784-5792.
1990 American Chemical Society
Oxidation Number Applied to Organic Compounds is expected, contour maps of electron density distribution are drawn to analyze the contributions both from u- and a-electrons. Method of Calculation The spherically averaged electron density, po(R),and its increment relative to the summed-up values of the component atoms, Apo(R),are defined as'
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 2821 TABLE I: Apo((r2)'/2)Values around the Nitrogen Atom with MIDI-4 Basis and the Modified Oxidation Numbers molecule A ~ ~ ( 0 . 6 3A)4 X IO3 mod oxidn no. ref"
N H3 NH2NH2 NH=NH N2
12.0 8.9 1.9 -1.6
-9 1 4 -3 12 -314 0
16
16 16 17
" Reference for the molecular geometry.
atom
APO(R)= PO(R)-
E Poi(R) 1
where N ( R ) is the number of electrons in a sphere with radius R, and the subscript i refers to the contribution of the component free atom i . The deformation electron number A N ( R ) is also defined as
A N ( R ) = N ( R ) - a?Ni(R) i
The analytical expressions for these quantities using Gaussian-type functions (GTF) were obtained by Iwata.6 The central position of the sphere for calculating these quantities can be moved to any point if necessary. I n this paper the radius dependency of the Apo(R)values is mainly examined. The geometries of the compounds studied were taken from the experimentally determined datai6*''and/or optimized parameters with ab initio calculations for the singlet ground We have extensively studied the basis set dependency of these quantities, but in this paper only the results of the analysis of Apo(R) at the R H F level with the MIDI-4 basis of double zeta (DZ) quality proposed by Tatewaki and H ~ z i n a g are a ~ ~given. The electron number analysis was performed for the component atoms of 26 acyclic organic compounds containing C, H, 0, and N atoms, with the hydrogenated nitrogen compounds as the references. Further, the density maps were drawn to examine the angular dependency of electron density and the contribution of u- and a-electrons to electron distribution around atoms. RKNGAUSS and SCMOLX program packages by Iwata's group of Keio University were used after a slight modification so that the u- and a-electron distributions are separately calculated. The computers used in this work are the HITAC M680H at the Institute for Molecular Science, the HITAC M682H/M680H at the University of Tokyo, and the work station Sony NEWS821 in our laboratory. Oxidation State of Hydrogen Atoms According to our recent studies the oxidation number of the hydrogen atom in a C H bond can be assigned + 1 /2 rather than + I of that in an O H bond. In this study the Apo around the hydrogen atom in various nitrogen compounds were calculated and their values at the bonding region 0.5-0.8 A were found just between those of C H and OH. Figure 1 shows the radial de(16) Landolt-Bornstein, New Series 11-7. Structure Data of Free Polyatomic Molecules; Springer-Verlag: West Berlin, 1976. (17) Huber. K. P.; Herzberg, G. Molecular Spectra and Molecular Structure. In Constants of Diatomic Molecules; Nostrand Reinhold: New
York. - - - - - ,1969. ( I 8) Van Alsenoy, C.; Scarsdale, J. N.; Williams, J. 0.: Schafer, L. J . Mol. Struct.: THEOCHEM 1982,86, 365-376. (19) Schafer, L.;Van Alsenoy, C.; Williams, J. 0.; Scarsdale, J. N.; Geise, H. J. J. Mol. Struct.: THEOCHEM 1981, 76. 349-361. (20) Van Alsenoy. C.; Scarsdale, J. N.; Williams, J. 0.; Schafer, L. J . Mol. Struci.: THEOCHEM 1982, 86, 291-295. (21) Eades, R. A.; Weil, D. A.; Ellenberger, M. R.; Farneth, W. E.; Dixon, D. A.; Douglass, Jr., C. H. J . Am. Chem. SOC.1981, 103, 5372-5377. (22) Muller, K.; Brown, L. D. Helu. Chim. Aria 1978, 61, 1407-1418. (23) Pokier, R. A.; Majlessi, D.; Zielinski, T. J. J. Compur. Chem. 1986, 7, 464-475. (24) Tatewaki, H.; Huzinaga, S. J. Comput. Chem. 1980, I , 205-228.
TABLE 11: Apo((r2)'/2)Values around the Oxygen and Carbon Atoms with MIDI-4 Basis and the Modified Oxidation Numbers' molecule Apo((r2)'/2)bX IO3 mod oxidn no. H2Q 19.5 -2 H2Q2 11.1 -1 CH4 CH3CH3 CH2=CH2 CH3OH HCHO HC02H c02
9.8 8.7 7.8 4.4 -1.7 -6.9 -14.9
-2 -3 12 -1 -1 1 2 1
512 4
"Reference 2. b ( r 2 ) 1 / 2values for oxygen and carbon atoms are 0.590 and 0.702 A, respectively.
pendency of Apo values around the hydrogen atoms in the OH, NH, and CH bonds of aminomethanol and the reference molecules, H 2 0 , NH,, and CH4. In our previous study, it was found that the Apo values in the bonding region from 0.5 to 1.O A show such quantitative information of the electron density distribution around the specified atom that gives quantum mechanical interpretation of the oxidation state. Let us concentrate our attention on the shapes of the Apo curves in that region. Since the larger the Apo value the lower the oxidation state of the atom, the observed stepwise change of the Apo curves along the OH, NI4, and C H bonds means that the oxidation state of the hydrogen atom becomes stepwise lower in this order. Thus the modified oxidation number of the hydrogen atom in N H bonds can be assigned as +3/4 just in the middle of 1 for OH and 1/2 for CH. This is clearly shown in Figure 1 for the Apo curves of three different types of hydrogen atoms in aminomethanol, where the OH, NH, and C H hydrogen curves almost coincide with the respective reference curves to give the following feature of this molecule
+
H+31L
\ H /N-
+
H+1/2
I C-
I
0-
Hi'
H
It seems to be reasonable if we consider that the Pauling electronegativity of nitrogen atom is 3.0 which is just the mean of the values of carbon (2.5) and oxygen (3.5) atoms. Further, detailed analysis of various types of XH bonds (X = H, B, C, N, 0, F, Si, P, S, CI) with different polarity also justifies this modification of the oxidation number of the hydrogen atom.2s Reference Molecules for the Assignment of Oxidation Numbers The sum of the oxidation numbers of all the component atoms should be zero for neutral molecules. Then, the oxidation numbers of the nitrogen atoms in the series of hydrogenated nitrogen compounds can be uniquely assigned as -9/4, -3/2, -314, and 0, respectively, for ammonia (NH,), hydrazine (NH2NH2),diimide (NH=NH), and nitrogen (N2), since the value of +3/4 for the hydrogen atom in N H bond is found to remain constant. This tendency is really in parallel with the observed stepwise change of the Apo curves of N atoms in the bonding region (see (25) Takano, K . Doctoral Thesis, Osaka City University, 1988.
Takano et al.
2822 The Journal of Physical Chemistry, Vol. 94, No. 7, 1990
C
_--
H20
0N
+1
-
NH3
--- cy4
I
-
'112
NH2CHzOH NH2CHzOH --e* NHzCH20H --+-
I
I
-
I
Figure 3. Difference spherically averaged electron density Ap,,(R) around the carbon atom in methylhydrazine, aminomethanol, and the reference molecules (unmarked lines) with the MIDI-4 basis. The right-hand
values of chemical formula are modified oxidation numbers. Newly assigned values are shown in parentheses.
Aa0 I R I
4 i
0
.
Ti;
ftl ( - 312)
H2Q2
0.0
0.5
1.0
1.5
RIi1
Figure 4. Difference spherically averaged electron density Appo(R) around the oxygen atom in 2-aminoethanol,aminomethanol, hydroxylamine, and the references H20and H202(unmarked lines) molecules with the
MIDI-4 basis. The right-hand values of chemical formula are modified oxidation numbers. Newly assigned values are shown in parentheses. numbers. Hereafter we choose these unmarked curves in Figures 1 and 2 and the A P ~ ( ( ? ) ~ /values ~) in Tables I and I1 as the references to assign the oxidation numbers of various oxygen, nitrogen, and carbon atoms. In what follows, the results of the electron number analysis applied to a series of organic compounds containing C, H, 0,and N atoms are presented to show the validity of our assignment and the novel additivity relationship among the modified oxidation numbers. Saturated Organic Compounds Containing Nitrogen Atoms Figures 2, 3, and 4, respectively, show the radial dependency of Ap, values around nitrogen, carbon, and oxygen atoms in various saturated compounds. The curves for the reference molecules are
Oxidation Number Applied to Organic Compounds
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 2823 1 I
'
,' 0 ; C -N , , I
/ '
0
-1
H-@-H H2
-2
Figure 5. Correlation between the observed Apo value and the assigned oxidation number for nitrogen atoms in the saturated and unsaturated organic compounds listed in Tables I, 111, and IV. The symbols are as A) unsaturated comfollows: ( 0 )saturated compounds in Table I; (0, pounds, respectively, belonging to groups A and B in Table I l l ; ( X ) reference molecules in Table 1.
also given for comparison. First consider methylhydrazine. The oxidation numbers of the heavy atoms could be assigned as C-'H3N-S/4HN-3/2H2,from the comparison of the Apo curves in the bonding region (0.5 5 R 5 1 .O A). That is, for the carbon and terminal nitrogen atoms the Apo curves are very similar to their corresponding reference curves of C-'H2=CH2 and N-3/2H2NH2as shown in Figures 2 and 3. The oxidation number (-5/4)of the central N atom was interpolated from the two reference compounds, N-3/4H=NH and N"12H2NH2 (see Figure 2). Note that by combining these values with the already assigned values of hydrogen atoms (CH+1/2and N13+3/4)the total sum of the oxidation numbers of this molecule turns out to be zero. ,412
+I12
H-C
I -1
-N
I -5/4
i-ry4 - 3 w
-N,
,$lR
Next consider aminomethanol. The oxidation states of three different H atoms were already unambiguously assigned as CH+'i2, NH+3/4,and OH+' (see Figure I ) . The Ap,, curve for t h e 0 atomalmost collapses with that of the reference H 2 0 molecule and gives the normal oxidation number of -2 (see Figure 4). As the Apo curve for the terminal N atom lies just below the curve Of 13-9/4H3and far up N-3/2H2NH2as shown in Figure 2, we tentatively put the number -2 for this N atom. For the C atom of N H 2 C H 2 0 H the curve in Figure 3 lies in between those of C-'/2H30H and HC'IHO. It is very difficult to decide whether we take or solely from this information. However, the latter choice would make this molecule formally electroneutral as H+3/4
H*112
\ N-2- C - 0 -'rV2 H +3/4/ H
-2
+I
I
Ht112
and we take this assignment. There happens to arise some ambiguity in assigning the oxidation number like the above case. However, we could carefully perform cross-checking not only for the whole Apo curves and Ape( values within a single atomic species but also for the attainment of electroneutrality within each molecule. All the assigned values given in Tables I-IV are thus guaranteed as they
Figure 6. Arrangement of the assigned oxidation numbers of carbon atoms in the organic compounds containing C, H, 0, and N atoms.
stand. Figure 5 illustrates how good correlation can be seen between the observed A p o ( ( S ) 1 / 2 )value (taken from Tables Ill and IV) and the assigned oxidation number for nitrogen atoms studied. The correlation coefficients for all the nitrogen compounds are obtained to be as high as 0.98. For oxygen and carbon atoms the values of the correlation coefficient were obtained to be almost the same as this. Thus we believe that possible error for the assigned oxidation number is less than a quarter within our scale. Now let us take a global view of the assigned oxidation numbers for each atomic species in saturated compounds separately. For the oxygen atoms in saturated alcohols, the oxidation state was found to be the same as that of water molecule. However, for hydroxylamine the Apo curve around oxygen atom lies just in between those in H B - 2 and HD2-I (see Figure 4). Thus for these molecules we can assign the oxidation number for oxygen atom as N H 2 C H 2 C H Q 2 H ,NH2CH@-2H, and NH,0-3/2H. This means that only a small fraction of electron is attracted from the nitrogen atom to the oxygen atom in the hydroxylamine as a consequence of the slight difference in the electronegativity of these two atoms. The oxidation number of carbon atoms given in Table Ill widely range from -3/2 to +1/2. We can arrange the assigned oxidation numbers of carbon atoms systematically as in Figure 6 . By following the broken lines we can see various series of compounds where the H atom attached to the central carbon atom is substituted with a successively electronegative group. If we take the series CHI CH3CH3 CH3NH2 C H 3 0 H , the oxidation numbers of C atom is found to increase stepwise from -2 to -1/2. No matter what the oxidation number of the C atom of the first CH3 N H 2 member is, each step up along the series H O H donates +1/2 to the oxidation number of C atom without exception. Thus we can predict the oxidation numbers of C in the hypothetical CH2(0H)2molecule to be + 1 .26 It is also found that the oxidation state of the terminal methyl C atom can be determined by its directly bonded group as exemplified in the two series of compounds C-3/2H3CH3 C-IH3NH2 C-1/2H30H and C-3/2H3CH3 C-3/2H3CH2NH2 C-3/2H3CH20H. For the latter series the oxidation number of the terminal C atom remains to be constant. The assigned oxidation numbers of N atoms can also be arranged in Figure 7 . Substitution of CH3 by N H 2 and O H similarly increases the oxidation number of N atom stepwise by 1 /2. However, a step up from H to CH, gives only 1/4 unit to the oxidation number of the attached N atom as is clearly seen
-
-
-
- -
-
+
-
+
+
(26) Because of the possible 'anomeric effect" between the two lone pair electrons in the electronic state of this molecule might not simply be estimated. However, we think that this effect is not as large as that of conjugated *-systems.
2824 The Journal of Physical Chemistry, Vol. 94, No. 7, 1990
TABLE 111:
Takano et ai.
Apa((rZ)’/Z) Values with MIDI-4 Basis and the Modified Oxidation Numbers (in Parentheses) for 0, N, C, and H Atoms in
S.tunted compounds
x 103 0 0.590A
comwund a a’ b b’ CH3NHz
C 0.702A
N
0.634A
b, 11.1 (-2)
H 0.843A
rep
a, 6.6(-1)
a’, -2.9b(1/2) b’, -6.8(3/4)
16
a. 10.0 (-3/2) b, 5.9(-l/2)
a’, -2.8 ( 1 /2) b’, -2.7(1/2) c’, -6.1 (3/4)
18
a a’ b b’ c c’ CH3CHzNHz
C, 11.8
b b‘ a a’ NH(CH,)z
b, 10.1 (-7/4)
a, 7.1( - I )
a’, -3.0b( 1 /2) b’, -6.5 (3/4)
16
9.8(-3/2)
a, 7.3(-1)
a’, -3.0b (1 /2)
16
b, 12.3(-2)
a, 3.7(0)
a’, -3.8(1/2) b’, -5.9(3/4)
19
b, 6.7(-5/4) (-3/2)
a, 7.8(-1)
a’, -3.2b(1 /2) b’, -7.2(3/4) c’, -6.9(3/4)
16
c, 11.1 (-7/4)
a,
9.8(-3/2) b, 6.0(-l/2) d, 7.2(-1)
a’, -2.9 ( 1 /2) b’, -2.7(1/2) c’, -5.8 (3/4) d‘, -2.8 (1/2)
18
a, 11.1 (-2)
b, 5.7(-l/2)
a‘, -6.5 (3/4) b’, -2.8( 1 /2)
16
a‘, -8.7(3/4) b’, -8.1 ( I )
16
a’ b’ c’ CHjNHNH2 a b c
(-2)
C, 8.1
b, 15.4(-3/2)
a, 5.0(-1)
a a’ b b’ c c‘ NHzCHzOH
C, 19.3(-2)
a, 12.4(-2)
b, 1.5 (1/2)
a’, -5.9(3/4) b’, -4.4(1/2) c‘, -6.4( I )
19
a a’ b b’ c c’ d d’ NH$HZCHZOH
d, 19.1(-2)
a, 12.2(-2)
b, 6.9(-l/2) c,4.3(0)
a’, -6.1 (3/4) b’, -2.7(1/2) c’, -3.1 (1/2) d’, -6.8( I )
20
Reference to the molecular geometry. bAveraged value. CHART I1
@ N I=
fa) ,-,
i
-142 O I
oxidation number
@ ;();-
{
group oxidation number
-8-
N-2-
c-l/2
Hy‘4
H+I
(-;)
(+;)
-21 -
++
polarization
Figure 7. Arrangement of the assigned oxidation numbers of nitrogen atoms in the organic compounds containing C, H, 0, and N atoms.
CHART I
(a)
CHn-CHd 0
0
(b)
CH,-NH, +1/2 -1/2
(C)
CH,-OH +I
N H,-NH,,,, 0 0
NH,-OH +1/2 -112
-I
in the series of NH3 to N(CH,),. The oxidation number of the terminal amino N atom is also determined by its directly bonded groups.
{
group
oxidation number
c-213 -c-
0 +I--1
-11 2
N-7/ 4
(-1)
-C-’
H+3/4
Hg”’ (0) 0-
polarization
(+1)
0-
I-
oxidation number
0-*
H:It2
(++)
H;I2
(-1)
(+ 3)
0 I
+5---
1
2
_-21 -+ + These stepwise and regular changes of the oxidation numbers of C, H, 0,and N atoms were found quite easily to be predicted by the following set of simple rules. Let us take the sum of the oxidation numbers for an atomic group (CH,, NH,, or OH) and call it the group oxidation number. The pair of group oxidation
The Journal of Physical Chemistry, Vol. 94, No. 7 , 1990 2825
Oxidation Number Applied to Organic Compounds
TABLE IV: Apo((r*)’/z) Values with MIDI-4 Basis and the Modified Oxidation Numbers (in Parentheses) for 0, N, C, and H Unsaturated ComPounds
Atoms in
pp0 x 103
compound
0 0.590A
N
C
0.634 A
0.702A
H
0.843 A
reP
a’, -3.0’(1 /2)
16
Group A a a ‘ b cc’ CH>N=NH
b, 1.2 (-l/2) C,
aa’b C H 3N=N C H 3 a a‘ b b’ CHI=NH aa‘bb’ cc’ CH>CH=NH
a, 8.0 (-1)
c’, -10.0 (3/4)
3.2 (-3/4)
b, 2.0 (-l/2)
a, 7.2 ( - I )
a’, -2.8’ (1 /2)
16
b, 7.0 (-3/2)
a, 4.4 (-l/4)
a’, -3.5’ ( I /2) b‘, -8.7 (3/4)
21
7.9 (-3/2)
a, 10.3 (-3/2) b, 3.4 (I /4)
a’, -3.0b(1 /2) b’, -3.1 (1/2)
21
C,
c‘, -8.4 (3/4) a a‘ b b’ CHz=NCH3
6.2(-5/4)
a a’ b b’ c c’ CHJCH=NCH3 aa’ bb’ C H 2=N 0H a a‘ CH,N=O a a’ b b’ CH3CHO
7.5 (-5/4)
b, 14.9 (-3/2)
a, 4.7 (-l/4) b, 7.5 (-1)
a’, -3.4’ ( 1 /2)
a, 9.8 (-3/2) b, 3.7 (1/4) c, 7.5 (-1)
a’, -2.9b(I /2) b’, -3.0(1/2)
16
b’, -3.2’(1/2) 16
c’, -3.2‘ (1 /2)
0.2 (-l/4)
a, 5.6 (-l/4)
a‘, -3.5’ (1/2) b’, -8.2 (I)
16
-6.1 (1/4)
a, 8.7 (-1)
a’, -3.1’ (1/2)
16
15.3 (-3/2)
a, 9.4(-3/2) b, -2.4 ( I )
a’, -2.6b( 1 /2) b’, -3.0 (1 /2)
16
16.1 (-3/2)
a, 10.7(-3/2) b, -2.6 (3/2)
a’, -2.9’ ( I /2)
16
a, 9.2 (-3/2) b, 6.9 (-l/2) C, 8.5 (-I)
a’, -2.9’ (1 /2) b’, -3.0 (1 /2)
16
b, 3.5 (0)
a’, -6.6b (3/4) b’, -3.4 (1 /2)
8.1 (-3/4)
a a ’ b b ’ cc’ CHICH=CHz
c’, -3.1’ (1/2)
Group B a a ’ b b ’ cc’ N H IC H=C H2
a, 10.4 (-3/2)
C,
9.7 (-3/2)
21 22
c’, -2.6’ ( 1 /2) a a’ b b’ c c’ NHZCHxNH
a, 12.4 (-7/4) c, 11.4 (-7/4)
b, 0.2 (3/4)
a’, -5.4 (3/4) b’, -3.8 (1/2)
23
c’, -8.1 (3/4) a a’ b b’ NH2CHO
18.2 (-7/4)
a, 11.9 (-7/4)
a a‘ b c c’ NH2COCH3
19.0 (-7/4)
a, 11.6 (-7/4)
b, -4.5 (3/2) b, -5.4 (2) C,
8.7 (-3/2)
a’, -5.1b (3/4) b’, -4.I (1 /2)
16
a’, -5.6’ (3/4)
16
c’, -2.5’ (1/2)
*Reference to the molecular geometry. ’Averaged value. numbers for adjacent groups may be called bulk polarization. For the saturated compounds studied there are five different combinations of groups as shown in Chart 1. For all the atomic groups in the 1 1 saturated compounds in Table I l l the group oxidation numbers are found to be reproduced without exception by adding the assigned contributions of the group oxidation number in the bulk polarization attributed to the five kinds of bonds in Chart
I. Some examples are shown in Chart I1 for (a) 2-aminoethanol and (b) ethylmethylamine. The superscripts in Chart I1 are the oxidation numbers assigned from the Apo curves of individual atoms in these particular molecules. The figures in parentheses below each atomic group are the group oxidation numbers. These values are exactly reproduced by adding the contributions from the bulk polarization listed in Chart I. It is to be mentioned that the sum of the group oxidation numbers in each bulk polarization assigned in Chart I is equal to zero. Further, in the order of (a), (b), and (c), the magnitude of polarization changes stepwise in parallel with the order of the difference in the electronegativity
of the central atom in the group, Le., 0 (C-C and N-N), 0.5 (C-N and N-O), and 1 .O (C-O). The difference in the group oxidation numbers between the two neighboring atomic groups may represent the extent of polarization in the bond connecting these two groups. The fact that the magnitude of polarization between C and 0 is equal to the sum of those for C-N and N - O bonds makes things simple. If we use these rules, the modified oxidation number of the component atoms in a given molecule can be obtained just from its structural formula. These results are not only an extension of the additivity rules proposed in the previous paper for the compounds containing only C, H, and 0 atoms but also lead to the conclusion that the oxidation-reduction or electron-donating-electron-accepting phenomena among C, N, 0,and H atoms are governed by a very simple principle as long as saturated compounds are concerned. Unsaturated Organic Nitrogen Compounds Containing One Double Bond Our electron number analysis was also applied to 15 unsaturated
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The Journal of Physical Chemistry, Vol. 94, No. 7, I990
a 40I R I
Takano et al. CHART 111
r
1i
C & N
(a-1)
oxidation number group
oxidation
1
number +
$
--1
polarization
0-
0
(a-2)
oxidation
--b-
-+--e-
0.0
0.5
1
1.0
i
CH3ijHNH2 -514 CH3NzijH ( - 3 / L ) ! CH-jijzNH ( - 1 1 2 ) ’ I
number group oxidation number polarization
1.5
Rlil Figure 8. Difference spherically averaged electron density Ppo(R) around the carbon and nitrogen atoms in methyldiimide (marked lines) and the corresponding saturated compound, methylhydrazine (unmarked lines) with the MIDI-4 basis. The right-hand values of chemical formula are modified oxidation numbers. Newly assigned values are shown in parentheses.
nitrogen compounds by checking the radial dependency of the Apo curve in the bonding region, and the Apo((G)’/2)values are given in Table I V . For hydrogen atoms the analysis was performed mainly with the Ap0 curve. It was found that the oxidation state of the hydrogen atom depends solely on the kind of the bonded atom, as symbolized as CkJ+1/2,NH+3/4,and OH+’. According to the results of the assigned oxidation states of C, N, and 0 atoms, the unsaturated compounds studied are grouped into two, A and B, depending on the extent of intramolecular electron transfer. That is, the nitrogen atom adjacent to a double bond tends to donate its a-electron to the neighboring bond. We call the series of such compounds group B. First we will discuss the results of group A. The radial dependency of the Apo curves around the nitrogen and carbon atoms in methyldiimide (CH3N=NH) are compared with the curves of the corresponding saturated compound, methylhydrazine, as the references (see Figure 8). The curves for the carbon atoms in the methyl groups of the two compounds are very similar to each other in the bonding region from 0.5 to 1 .O A. This means that the oxidation number of the carbon atom of methyl group in methyldiimide can be assigned as -1, which is the same as in the case of the reference, C-IH3NHNH2. The N H hydrogen atom of methyldiimide is found to have the same oxidation number (+3/4) as that of saturated compounds. From the comparison of the four Ap0 curves of nitrogen atoms the oxidation numbers of = N H and -N= nitrogen atoms are extrapolated respectively to -3/4 and -1/2. These assignments obtained from the Apo analysis are shown to be consistent if we assume the bulk polarization of N=N double bond to be zero. These oxidationreduction features of methyldiimide are illustrated in Chart Ill(a-l), which is to be compared with the saturated compound, methylhydrazine, in Chart III(a-2). For the compound with a C=N double bond the result of methylenimine CH3N=CH2 is shown in Chart III(b-I) to be compared with the saturated compound, dimethylamine (CH3)2NH in Chart III(b-2). For group A one can deduce a simple regularity for the group oxidation numbers thus assigned. As clearly seen in Chart ll(a-l),(b-l), similar additivity rules of oxidation numbers adopted for saturated compounds can be extended to unsaturated ones by adding the contributions of the group oxidation numbers attributed to the bulk polarization in
CHART IV (a)
C H ,=C H , 0 0
NH,=NH,r 0 0
C H,=N H, +3/4 -314
NH,=O +3/4 -314
CH,=O +3/2 -312
(a-2)
the double bonds as given in Chart IV. By comparing Charts I and 1V one can infer that the degree of polarization of the heteropolar double bond is three halves of the corresponding single bond. Those compounds containing an amino group that is conjugated with a double bond form group B. The oxidation number of the nitrogen atom in the amino group adjacent to a double bond in these unsaturated compounds is different from that of the corresponding nitrogen atom in saturated compounds. This is contrary to the case of the carbon and oxygen atoms in the atomic
Oxidation Number Applied to Organic Compounds
The Journal of Physical Chemistry, Vol. 94, NO. 7, 1990 2827 group adjacent to a double bond in the compounds of group A. Chart V shows the assigned oxidation numbers of the compounds belonging to group B and the corresponding saturated compounds. It can be understood that the nitrogen atoms in the amino group in the former are a little more oxidized than those in the latter; that is, the values of the assigned oxidation number, -3/2 of nitrogen in vinylamine (&H2CH=CH2), is higher than the value -2 in ethylamine (&H2CH2CH3). Accordingly, the group oxidation numbers of the amino groups are higher for unsaturated than for saturated compounds. This is explained by the electronic migration from the amino group to the electron-withdrawing double bond in group B compounds. To differentiate between the degree of electron migration of U- and s-electrons, the deformation electron densities of formamide in group B and acetaldehyde in group A were drawn in the molecular plane and the one parallel to it (see Figure 9). I t is remarkable that the o-electron distribution around the nitrogen atom in formamide decreases more than that around the carbon atom of methyl group in acetaldehyde, while around the carbonyl groups of these two compounds the contour map of the deformation electron density is very similar in the molecular plane as shown in Figure 9a,c. On the other hand, regarding s-electron distribution, considerable difference is observed totally between the two compounds (see Figure 9b,d). In acetaldehyde the bonding s-electrons are localized between the carbon and oxygen atoms of the carbonyl group and the electron density decreases in the region around the oxygen atom, especially on the opposite side of the bond. On the contrary, r-electrons in formamide are seen to migrate from the amino group to the carbonyl group. In the compounds belonging to group B the increment of the s-electrons contrary to the decrement of the cr-electronsaround nitrogen atom is a general feature shown in the difference maps. Thus electron migration from the amino group in the conjugated molecules belonging to group B originates in u-electrons.
I
ti%
x
HH Ha , C I
C
H' ' 0
\
d
\
'IC Figure 9. Contour plots of the MIDI-4 deformation electron density of
(a, b) formamide and (c, d) acetaldehyde in the plane of the carbon, nitrogen, and oxygen atoms for u-electrons (a, c) and in the plane parallel to it with 1 .Oau above the molecular plane for r-electrons (b, d). Positive contours (electron excess) and zero contours are drawn as solid lines, and negative contours (electron deficiency) are dashed. The contours except for zero show the values of f0.008 X 2" and f 0 . 0 0 2 X 2" ( n = 0, I , 2, ...) e/au3, respectively, for u- and r-electrons. The positions of atoms in the plane are denoted by 0, and the projections of atoms into the plane are denoted by X .
Concluding Remarks For organic compounds consisting of nonequivalent carbon and nitrogen atoms, the assignment of the oxidation number is given by the aid of the Apo curves in the bonding region. The absolute values of these modified oxidation numbers are just the formal numbers as they stand, but they surely reflect not only the subtle but also the stepwise polarization of valence electrons in those carbon compounds and the oxidation states of atoms can be understood by the same scale throughout both the inorganic and organic compounds. The modification of the oxidation number of the hydrogen atom as Cl-J+'/2 and NH+3/4corresponding to OH+' led us to a systematic understanding and yielded simple additivity rules for the group oxidation numbers in the organic molecules containing nitrogen atoms. Therefore, the oxidation states of the C, N, and 0 atoms in saturated organic compounds can easily be predicted just from the group oxidation numbers by using the additivity rules of the bulk polarization between the neighboring atomic groups and the assigned oxidation numbers of hydrogen atoms as mentioned above, if the structural formula is given. Some regularity was found in the change of the oxidation state from the saturated to unsaturated compounds. For double bonds in nitrogen containing compounds similar bulk polarizations can also be introduced, if appreciable electronic migration is not expected to take place. Further, the extent of the electronic migration can be realized qualitatively by the contour maps of o- and s-electron density distribution for the system with an amino group neighboring to a double bond. Acknowledgment. We thank Professor Suehiro Iwata of Keio University for his helpful advice on our calculations. We also thank the Computer Center, Institute for Molecular Science, Okazaki National Research Institute, for the use of the HITAC M680H computer. Registry No. NH3, 7664-41-7; H2NNH2, 302-01-2; HN-NH, 3618-05-1 ; N2. 7727-37-9; MeNH2, 74-89-5; EtNH,, 75-04-7; NHMe2, 124-40-3; NMe,, 75-50-3; CH2(NH2)2,2372-88-5; MeNHNH2, 60-34-4;
J . Phys. Chem. 1990, 94, 2828-2832
2828
EtNHMe, 624-78-2; H,N(CH2)2NH,, 107-1 5-3; NH,OH, 7803-49-8; N H 2 C H , 0 H , 3088-27-5; N H 2 C H 2 C H 2 0 H , 141-43-5; MeN=NH, 26981-93-1; MeN=NMe, 503-28-6; N2C=NH, 2053-29-4; H,CCH= N H . 20729-41-3; H2C=NMe, 1761-67-7; MeCH=NMe, 6898-67-5;
H,C=NOH, 75- 17-2; MeN=O, 865-40-7; MeCHO, 75-07-0; MeCOMe, 67-64-1; MeCH=CH,, 1 IS-07-1; H2NCH=CH2, 593-67-9; H,NCH=NH, 463-52-5; NH,CHO, 75-12-7; NH,COMe, 60-35-5; C H 2 ( 0 H ) , . 463-57-0.
Magnetic Circularly Polarized Luminescence of Zinc Phthalocyanine in an Argon Matrix David H. Metcalf,+ Thomas C. VanCott,? Seth W. Snyder,$ Paul N. Schatz,*Vt Chemistry Department and Biophysics Program, University of Virginia, McCormick Road, Charlottesville, Virginia 22901
and Bryce E. Williamson Chemistry Department, university of Canterbury, Christchurch I , New Zealand (Received: July 19, 1989; I n Final Form: October 16, 1989)
The magnetic circularly polarized luminescence (MCPL) of the Q transition of zinc phthalocyanine has been measured in an argon matrix. These are the first such measurements for a matrix-isolated sample and support previous interpretations that require a crystal-field-stabilized Jahn-Teller splitting in the 'E,(n*) excited state. A moment analysis indicates the presence of a Ham effect which reduces the orbital angular momentum of this state by about 30%. Energy transfer between inequivalent sites in the matrix results in a red shift of the emission relative to the Q(O.0) absorption band.
Introduction Recently we reported detailed absorption and magnetic circular dichroism (MCD) spectra of zinc phthalocyanine in an argon matrix (ZnPc/Ar) between 14700 and 74000 cm-l.] (See ref 1 and references therein for recent reviews of the spectroscopic literature on Pc's.) The lowest energy allowed transition of ZnPc, at 15 200 cm-I, is designated the Q transition and in D4* symlE,(x*). metry corresponds to 'Alg(?) MCD' and emission studies2 have shown that the degeneracy of the Q state is lifted in a matrix, probably as a result of a crystal-field-stabilized Jahn-Teller effect.',2 One of the consequences of this splitting is that the magnetic circularly polarized luminescence (MCPL) from the Q state should consist purely of negative B terms (see later). In this work, we report the emission and MCPL of ZnPc/Ar obtained using a sensitive CPL spectrometer that was designed by one of us (D.H.M.) and built at the University of Virginia. We believe that these are the first reported measurements of MCPL from a matrix-isolated sample.
-
-
Experimental Section The full procedure for matrix deposition has been described p r e v i o ~ s l y . ' ~The ~ ZnPc, purified as previously described,' was sublimed at -34OOC and codeposited with argon on a cryogenically cooled (-5 K ) sapphire window in the bore of a superconducting coil. The initial spectra were unresolved, the Q(0,O) band consisting of a single almost symmetrical absorption contour of -75 cm-' fwhm accompanied by a corresponding single positive A term. However, annealing the spectra for a few minutes at -20 K produced a dramatic sharpening with the emergence of perhaps 10 distinct sites (solid curves, Figures 1 and 2). The spectra are quite similar to those obtained in our earlier study (ref I , Figure 5) but are even more clearly resolved. MCD and double-beam absorption spectra were obtained simultaneously using a spectrometer that has been described earlier.4,s The magnetic field strength and spectral resolution were respectively 0.37 T and 0.15 nm. The depolarization of circularly polarized light due to the sample was found to be negligible by comparing the natural C D of a solution of A-tris( 1,2-ethanedi'Chemistry Department, University of Virginia. *Biophysics Program, University of Virginia.
0022-3654/90/2094-2828$02.50/0
amine)cobalt(III) inserted after the sample with that of the same solution in the absence of the sample. The total luminescence (TL) and the MCPL were measured with an instrument that will be described in detail later.6 Emission was excited with 488-nm radiation from an argon ion laser (Coherent Radiation CR-5) which was focused to a slit image at the sample. The emitted light was collected at 180' to the incident beam and passed through a fused-quartz photoelastic modulator and a 570-nm glass cutoff filter to the entrance slit of a 3/4-m double monochromator (Spex 1400-2). The spectral resolution was 0.3 nm. The luminescence was detected in photon-counting mode by using a cooled photomultiplier tube with S-20 response. The MCPL was determined with a laboratory-built gated photon counting device referenced to the photoelastic modulator.6 The MCPL measurements were made at a field strength of 3.00 T and at a temperature of -5 K. Results The spectra over the full range of the Q-band region are illustrated in Figure l . The absorption ( A ) and MCD (AA) spectra (solid lines) are respectively compared with the TL ( I ) and MCPL (AI) shown by the dashed lines. Three major band envelopes are observed in absorption. These are denoted Q(O,O), Q(l,O) and Q(2,O) in Figure I . Q(0,O) comprises the zero-phonon bands of several sites.] Q( 1,O) and Q( 2,O) contain contributions from overlapping vibronic but Q(2,O) also contains a contribution from a separate electronic transition.'*2 To the red of the Q(0,O) region in emission are two band envelopes, at -14500 and -13700 cm-I (Figure 1). These contain vibronic bands associated with the Q transition of ZnPc'** but also contain a substantial contribution due to small amounts of H2Pc impurity, as has been observed by previous workers.2 ( I ) VanCott, T. C.; Rose, J. L.; Misener, G. C.; Williamson, B. E.; Schrimpf, A. E.; Boyle, M. E.; Schatz, P. N. J . Phys. Chem. 1989.93,2999. (2) Huang, T. H.; Rieckhoff, K. E.; Voigt, E. M. J . Chem. Phys. 1982, 77, 3424. (3) Lund, P. A.; Smith, D.; Jacobs, S. M.; Schatz, P. N. J . Phys. Chem. 1984, 88, 3 1 . (4) Rose, J.; Smith, D.; Williamson, B. E.; Schatz, P. N.; O'Brien, M. C. M. J. Phys. Chem. 1986, 90, 2608. ( 5 ) Misener, G. C. Ph.D. Dissertation, University of Virginia, Charlottesville, VA, 1987. ( 6 ) Metcalf, D. H.; Cummings, W . J.; Richardson, F. S. To be published.
0 1990 American Chemical Society
