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A HMO study of the electronic spectra of some substituted hexatrienses
JOURNAL
OF MOLECULAR
24 (1967)
SPECTROSCOPY
NOTES A HMO Study of the Electronic Spectra Some Substituted Hexatrienses
of
The molecular orbital theory as applied to organic chemistry has made considerable progress since Hiickel proposed his simple LCAO-MO method in 1931 (1). Nonetheless, the simple theory, which provides answers in terms of one or two parameters, still remains useful as a t,heoretical framework for correlating the properties of the members of a given series of organic compolunds. The purpose of the present communication is to report the study of the electronic spectra of a series of methyl- and chloro-substituted 1,3,5 hexatrienes within the formalism of the simple Iliickel MO thoery. TABLE PARAMETERS
Carbon Methyl (i) Conjugation model (--c-C,~B,) (ii) Inductive model (-C-Me) Chloriue
I IX THIS
USED +
hxPc_c
ax
=
ac
(30-x
=
Kc_xPc_c kc-c (long) kc-c (short)
hc = 0 hca = h3 =
STULJY
-0.1 -0.5
hc = -0.5 hc, = 2
= 1 = 1.25
kC_Ca = 0.8 kCsH3 = 3 kc-Ale = 0 kc_c, = 0.4
The details of preparat,ion and the X,nax of the molecules st,udies in the present invest,igation have been reported by us elsewhere (2). Figures 1 and 2, on the other hand, give the hitherto unpublished electronic spectra of these molecules. As usual, the first t#ask in the theoretical analysis was the determination of a common exchange integral p from the electronic spect’rum of the unsuhstituted hexatriene. Thus, using the reported value of 39 216 cm-1 for the first electronic excitation energy (S), as well as a ratio of b(short bond)/p(long buud) of 1.25, as suggested by Murrell et al. (4) for the study of the electronic spectra of polyenes, we calculated P to be 29 619 cm-l. The latter value, combined with the “recommended” parameter values given by Streitweiser (5) for heteroatoms, enabled us to calculate the first electronic excitation energies of the molecules studies. Table I sltmmarizes the values of the parameters used in the present investigation, while Table II gives the observed spectral “centers of gravities,” as well as t,he calculated values of the said excitations. An esaminat,ion of Table II reveals that in both series the agreement between experiment and theory is rather encouraging. It should be noticed, however, that in the met,hyl-sub244
NOTES
0.8_
W 0 0.16_ z Q m : (I) In a 0.a4_
0.: 2_
O.(I, 210
230
FIG. 1. Near uv spectra of methyl-substit,ut.ed 1,3,5 (1.90 X 10-%I) ---, (ii) 2-iMe (I .56 X IO-%!l) -------, TABLE OIMERVED AND ~~~~~~~~~~~~~“FIRST” Obs
290
270
250 mP
hexatrienes in isooctane. (i) l-Me (iii) 3-1Lle (2.49 X IO-%I) .....
II ELECTRONIC EXCITATIONS (in cm-l)
Conj
Calc Ind 39 186 38 979 39 245
1 -Me 2JIe ?-Me
38 462 38 986 38 388
38 4i5 38 979 38 594
l-Cl 3-Cl 3-U
37 736 38 986 38 095
-~
38 751 39 067 38 860
NOTES
246
0.t
W
u 0.e
z a
m K
0 (I)
m a 0.4
3.2
_.-
.’
*a.*
0.0 210
250
230
270
290
mv
FIG. 2. Near uv spectra of chloro-substituted 1,3,5 hexatrienes in isooctane. (ii) 2-Cl (2.10 X lo-%I) --------, (iii) 3-Cl (1.18 X lO+M) (1.24 X 10-m) -, TABLE NET
ATOM CHARGES
Cl
CP
(i) I-Cl
.....
III
OF THE SUBSTITUTED
C3
HEXATRIENES
CC
CA
CS
l-Me 2-Me 3-Me
0.00659” -0.00994s 0.00111
-0.00984b 0.00334a -0.00055
0.00110 -0 00055 +0 :00466’
-0.00425 0.00053 -0.00879b
0.00018 -0.00005 0.00052
-0.00231 0.0018 -0.00426
l-Cl a-c1 3-Cl
0.01715” -0.0228Oh 0.00257
-0.02267b 0.00928” -0.00139
0.00256 -0.00139 0.01238”
-0.00961 0.00123 -0.02616b
0.00041 -0.00013 +0.00122
-0.00520 0.00041 -0.00963
* Probable b Probable
nucleophilic electrophilic
sites. sites.
NOTES stituted series ihe conjugation model seems to be a more reasonable model of lqperconjugation (6) than the inductive model of Wheland and Pauling (7). In fact,, the induct,ive model result,s in the calculat,ion of opposite trends than experimentally observed. Finally, Table III gives our cnlculntious of the uet atom charges at the nuclei lmdel cousideration (see illsert, on Fig. 1). An examinatiou of this table shows, for instance, that in the I-Me hexatriene, an clectrophilic substitution would be expected t,o take place at carbon 2, while a nucleophilic substitution would t,ake place at carbon 1. The reactivities of the other substituted hexat,rienes are indicated in Tahle III. REFERENCES 1% H~XEL, %. Physik 70, 204 (1931); 76, 628 (1932). C. W. SPINGLER .UVDG. FORNES,T WOODS, J. Org. Chenz. 30, 2218 (1965). J. C. II. lIw.1, P. L. DE BENNET-ILLE, .UVD11. J. SIMS, J. ;lrn. Chem. Sot. 82, 253i 1~1960). J. N. MURRELL, S. F. A. KETTLE, MD b. hl. TEDDER, “Valance Theory,” p. 285. Wiley, Kew York, 1965. 5. A. STILEIXVIESER, “1Iolecular Orbital Theory for Organic Chemists,” p. 135. Riley, New York, 1961. 6. 11. 8. MULLIBEIY, C. 8. RIEU, .\KD W. G. BROWN, J. _4m. Chena. Sot. 63, 41 (1941). 7. (;. w. WHEL\SD .\Nr) I,. PAULISG, d. Am. Chem. sot. 57, 2086 11935).
1. .?. 5. 4.
Departme&
0s C’heatistry,
WALTER
A.
lT13~~N0t3
AXD CH\RLES W. HP.\SGLER
.ILichael Fnrntlay Laboratories, _\-orthern Zllinois ITniversity, Deb-alb, Illinois 60115 Received JLa?y 27: 1967
The 3700-k
Electronic Transition and Some Parameters of Pyridazine*
Geometric
The first, published information ahout the geometric parameters of the pyridazine f.1,3di:~znbennene) molecule was derived from the longest-wavelength ultraviolet ahs)rpt,ion (1). One rotational constant. B = iA0 + B,)/2 was oht,ained for each of the electronic states dlB, and ,Tlil, . Since then, t.wo theses about the microwave spectrum have appeared (8, 5). Three independently-derived values of i? ,, for the ground state agree to within +O.OOOZcm-l, that is to O.l“;, and this agreement supports our use of elect,ronic spectra as a source of rotational constants (1). Werner’s work (3.1 gave enough constants to determine the ring geometry for the _y’Al (ground) state, namely, ,(X;,-Ns) = 1.330 .i. < NCC’ = 12.3.7” = 1.375 .I, Q CKN = 119.0”, r(C-C,) = 1.381 AK, r(c,-C>, and Q CCC! = 117.3”. (The distance ( i3-C6 = 2.64 s measures the lengt,h of t,he ring whili 0 t,he hreodth is X,-C5 = 2.41 11). Ilere, we combine B-values derived from the electronic spectra of pyridazine-ha, -d4, and 3,6-d? with the microwave results to obtain (1) coordnates for the hydrogen atoms and (2) separately, and more xccurstely, the c/lunge of hydrogen atom positions on electronic excitation. Point, group (‘2, is assumed for earl1 clectronic st,at,e. * Based on the Honors thesis of R. M. Lucas, Jr., filed in the library versity, 1967.
of Vanderbilt
I’ni-
OF MOLECULAR
24 (1967)
SPECTROSCOPY
NOTES A HMO Study of the Electronic Spectra Some Substituted Hexatrienses
of
The molecular orbital theory as applied to organic chemistry has made considerable progress since Hiickel proposed his simple LCAO-MO method in 1931 (1). Nonetheless, the simple theory, which provides answers in terms of one or two parameters, still remains useful as a t,heoretical framework for correlating the properties of the members of a given series of organic compolunds. The purpose of the present communication is to report the study of the electronic spectra of a series of methyl- and chloro-substituted 1,3,5 hexatrienes within the formalism of the simple Iliickel MO thoery. TABLE PARAMETERS
Carbon Methyl (i) Conjugation model (--c-C,~B,) (ii) Inductive model (-C-Me) Chloriue
I IX THIS
USED +
hxPc_c
ax
=
ac
(30-x
=
Kc_xPc_c kc-c (long) kc-c (short)
hc = 0 hca = h3 =
STULJY
-0.1 -0.5
hc = -0.5 hc, = 2
= 1 = 1.25
kC_Ca = 0.8 kCsH3 = 3 kc-Ale = 0 kc_c, = 0.4
The details of preparat,ion and the X,nax of the molecules st,udies in the present invest,igation have been reported by us elsewhere (2). Figures 1 and 2, on the other hand, give the hitherto unpublished electronic spectra of these molecules. As usual, the first t#ask in the theoretical analysis was the determination of a common exchange integral p from the electronic spect’rum of the unsuhstituted hexatriene. Thus, using the reported value of 39 216 cm-1 for the first electronic excitation energy (S), as well as a ratio of b(short bond)/p(long buud) of 1.25, as suggested by Murrell et al. (4) for the study of the electronic spectra of polyenes, we calculated P to be 29 619 cm-l. The latter value, combined with the “recommended” parameter values given by Streitweiser (5) for heteroatoms, enabled us to calculate the first electronic excitation energies of the molecules studies. Table I sltmmarizes the values of the parameters used in the present investigation, while Table II gives the observed spectral “centers of gravities,” as well as t,he calculated values of the said excitations. An esaminat,ion of Table II reveals that in both series the agreement between experiment and theory is rather encouraging. It should be noticed, however, that in the met,hyl-sub244
NOTES
0.8_
W 0 0.16_ z Q m : (I) In a 0.a4_
0.: 2_
O.(I, 210
230
FIG. 1. Near uv spectra of methyl-substit,ut.ed 1,3,5 (1.90 X 10-%I) ---, (ii) 2-iMe (I .56 X IO-%!l) -------, TABLE OIMERVED AND ~~~~~~~~~~~~~“FIRST” Obs
290
270
250 mP
hexatrienes in isooctane. (i) l-Me (iii) 3-1Lle (2.49 X IO-%I) .....
II ELECTRONIC EXCITATIONS (in cm-l)
Conj
Calc Ind 39 186 38 979 39 245
1 -Me 2JIe ?-Me
38 462 38 986 38 388
38 4i5 38 979 38 594
l-Cl 3-Cl 3-U
37 736 38 986 38 095
-~
38 751 39 067 38 860
NOTES
246
0.t
W
u 0.e
z a
m K
0 (I)
m a 0.4
3.2
_.-
.’
*a.*
0.0 210
250
230
270
290
mv
FIG. 2. Near uv spectra of chloro-substituted 1,3,5 hexatrienes in isooctane. (ii) 2-Cl (2.10 X lo-%I) --------, (iii) 3-Cl (1.18 X lO+M) (1.24 X 10-m) -, TABLE NET
ATOM CHARGES
Cl
CP
(i) I-Cl
.....
III
OF THE SUBSTITUTED
C3
HEXATRIENES
CC
CA
CS
l-Me 2-Me 3-Me
0.00659” -0.00994s 0.00111
-0.00984b 0.00334a -0.00055
0.00110 -0 00055 +0 :00466’
-0.00425 0.00053 -0.00879b
0.00018 -0.00005 0.00052
-0.00231 0.0018 -0.00426
l-Cl a-c1 3-Cl
0.01715” -0.0228Oh 0.00257
-0.02267b 0.00928” -0.00139
0.00256 -0.00139 0.01238”
-0.00961 0.00123 -0.02616b
0.00041 -0.00013 +0.00122
-0.00520 0.00041 -0.00963
* Probable b Probable
nucleophilic electrophilic
sites. sites.
NOTES stituted series ihe conjugation model seems to be a more reasonable model of lqperconjugation (6) than the inductive model of Wheland and Pauling (7). In fact,, the induct,ive model result,s in the calculat,ion of opposite trends than experimentally observed. Finally, Table III gives our cnlculntious of the uet atom charges at the nuclei lmdel cousideration (see illsert, on Fig. 1). An examinatiou of this table shows, for instance, that in the I-Me hexatriene, an clectrophilic substitution would be expected t,o take place at carbon 2, while a nucleophilic substitution would t,ake place at carbon 1. The reactivities of the other substituted hexat,rienes are indicated in Tahle III. REFERENCES 1% H~XEL, %. Physik 70, 204 (1931); 76, 628 (1932). C. W. SPINGLER .UVDG. FORNES,T WOODS, J. Org. Chenz. 30, 2218 (1965). J. C. II. lIw.1, P. L. DE BENNET-ILLE, .UVD11. J. SIMS, J. ;lrn. Chem. Sot. 82, 253i 1~1960). J. N. MURRELL, S. F. A. KETTLE, MD b. hl. TEDDER, “Valance Theory,” p. 285. Wiley, Kew York, 1965. 5. A. STILEIXVIESER, “1Iolecular Orbital Theory for Organic Chemists,” p. 135. Riley, New York, 1961. 6. 11. 8. MULLIBEIY, C. 8. RIEU, .\KD W. G. BROWN, J. _4m. Chena. Sot. 63, 41 (1941). 7. (;. w. WHEL\SD .\Nr) I,. PAULISG, d. Am. Chem. sot. 57, 2086 11935).
1. .?. 5. 4.
Departme&
0s C’heatistry,
WALTER
A.
lT13~~N0t3
AXD CH\RLES W. HP.\SGLER
.ILichael Fnrntlay Laboratories, _\-orthern Zllinois ITniversity, Deb-alb, Illinois 60115 Received JLa?y 27: 1967
The 3700-k
Electronic Transition and Some Parameters of Pyridazine*
Geometric
The first, published information ahout the geometric parameters of the pyridazine f.1,3di:~znbennene) molecule was derived from the longest-wavelength ultraviolet ahs)rpt,ion (1). One rotational constant. B = iA0 + B,)/2 was oht,ained for each of the electronic states dlB, and ,Tlil, . Since then, t.wo theses about the microwave spectrum have appeared (8, 5). Three independently-derived values of i? ,, for the ground state agree to within +O.OOOZcm-l, that is to O.l“;, and this agreement supports our use of elect,ronic spectra as a source of rotational constants (1). Werner’s work (3.1 gave enough constants to determine the ring geometry for the _y’Al (ground) state, namely, ,(X;,-Ns) = 1.330 .i. < NCC’ = 12.3.7” = 1.375 .I, Q CKN = 119.0”, r(C-C,) = 1.381 AK, r(c,-C>, and Q CCC! = 117.3”. (The distance ( i3-C6 = 2.64 s measures the lengt,h of t,he ring whili 0 t,he hreodth is X,-C5 = 2.41 11). Ilere, we combine B-values derived from the electronic spectra of pyridazine-ha, -d4, and 3,6-d? with the microwave results to obtain (1) coordnates for the hydrogen atoms and (2) separately, and more xccurstely, the c/lunge of hydrogen atom positions on electronic excitation. Point, group (‘2, is assumed for earl1 clectronic st,at,e. * Based on the Honors thesis of R. M. Lucas, Jr., filed in the library versity, 1967.
of Vanderbilt
I’ni-