Modeling substituent-dependence of the twist and shielding in a series of 4-substituted N-(4-nitrobenzylidene)anilines

Article abstract of DOI:10.1135/cccc20041566

A series of 15 4-substituted N-(4-nitrobenzylidene)anilines was synthesized and studied by ¹H NMR spectroscopy. Their ab initio calculated geometries and the shielding as expressed by aromatic ring currents were used in correlation analysis. The geometries were fully optimized using density functional theory B3LYP/6-311G** approaches. For the determination of the ring current contribution to the shielding of azomethine hydrogens H _α was used direct ab initio calculation of the chemical shielding in a model system. Experimental chemical shift values free of these contributions were successfully correlated with increments a_p of chemical shift for monosubstituted benzenes. In the same manner, the contribution of the anisotropy of C=N double bond to H_m hydrogen were calculated and values of the H_m chemical shift free of this contribution were successfully correlated with increments of chemical shift a_m.

Full text of DOI:10.1135/cccc20041566

1566
Proks, Holík:
MODELING SUBSTITUENT-DEPENDENCE OF THE TWIST AND
SHIELDING IN A SERIES OF 4-SUBSTITUTED
N-(4-NITROBENZYLIDENE)ANILINES
1,
2
Vladimír PROKS *,+ and Miroslav HOLÍK
Department of Theoretical and Physical Chemistry, Faculty of Science, Masaryk University,
1
2
Kotlářská 2, 610 37 Brno, Czech Republic; e-mail: proks@imc.cas.cz, holik@chemi.muni.cz
Received February 16, 2004
Accepted July 2, 2004
A series of 15 4-substituted N-(4-n itroben zyliden e)an ilin es was syn th esized an d studied by
1H NMR spectroscopy. Th eir ab in itio calculated geom etries an d th e sh ieldin g as expressed
by arom atic rin g curren ts were used in correlation an alysis. Th e geom etries were fully opti-
m ized usin g den sity fun ction al th eory B3LYP/6-311G** approach es. For th e determ in ation of
th e rin g curren t con tribution to th e sh ieldin g of azom eth in e h ydrogen s Hα was used direct
ab in itio calculation of th e ch em ical sh ieldin g in a m odel system . Experim en tal ch em ical
sh ift values free of th ese con tribution s were successfully correlated with in crem en ts ap of
ch em ical sh ift for m on osubstituted ben zen es. In th e sam e m an n er, th e con tribution of th e
an isotropy of C=N double bon d to Hm h ydrogen were calculated an d values of th e Hm
ch em ical sh ift free of th is con tribution were successfully correlated with in crem en ts of
ch em ical sh ift am
.
Keyw ord s: Ab in itio calculation s; Correlation an alysis; Azom eth in es; 1H NMR ch em ical
sh ifts; Im in es; Substituen t effects; Con form ation an alysis; NMR spectroscopy.
In recen t years an in creasin g in terest in polym er m aterials with good con -
ductin g1–3, fiber-form in g4, liquid-crystallin e5,6, an d n on -lin ear optical7,8
properties h as been observed. Som e of th ese m aterials are based on N-
ben zyliden ean ilin e m on om er un its th an ks to th eir elon gated an d π-con ju-
gated structure. For better prediction of th e beh avior of th ese polym er m a-
terials, an un derstan din g of th e relation sh ips between th e ch em ical struc-
ture an d ph ysical properties of th e m on om er m olecules could be useful9,10
.
Th e m ost im portan t feature of th e N-ben zyliden ean ilin e geom etry is th e
twist of th e an ilin e rin g from th e plan e of th e rest of th e m olecule. Th e
+ Presen t address: In stitute of Macrom olecular Ch em istry, Academ y of Scien ces of th e Czech
Republic, Heyrovskéh o n ám . 2, 162 06 Prague 6, Czech Republic.
Collect. Czech. Chem. Commun. (Vol. 69) (2004)
doi:10.1135/cccc20041566

4-Substituted N-(4-Nitrobenzylidene)anilines
1567
n on -plan ar con form ation s an d substitution depen den ces of th e deviation
of th e an ilin e rin g from th e C=N plan e bon d were studied by spectroscopic
m eth ods11,12, X-ray crystallograph y13, an d also by quan tum ch em ical calcu-
lation s14,15
.
An in terestin g ph en om en on is related to th e beh avior of azom eth in e h y-
drogen Hα ch em ical sh ift. It was observed th at wh en variable substituen ts
are situated in para position of th e ben zyliden e rin g16 (see Fig. 1, wh ere th e
n itro group is placed in th is particular position ), th e 1H ch em ical sh ift of
th e azom eth in e h ydrogen Hα can be correlated very well with th e Ham m ett
σp con stan ts17 of th e substituen ts an d with th e in crem en ts of th e ch em ical
18
sh ift ap
defin ed as a differen ce between 1H NMR ch em ical sh ift of a h y-
drogen atom in ben zen e an d in ben zen e substituted in para position rela-
tive to th is h ydrogen . Th ey reflect electron den sity ch an ges in particular
position relative to th e h ydrogen in question . Sin ce N=C group is bon ded
to th e sam e carbon atom as th e above-m en tion ed h ydrogen , th e ch an ge in
electron den sity on Hα is expected to be proportion al to th e ch an ge in th e
in crem en t ap. Th ese correlation s in dicate electron ic effects of th e substitu-
en ts. In con trast, th e correlation slope of th e Hα sign als in th e case of
4-substituted an ilin e rin g approach es zero16, wh ich could be erron eously
explain ed by an in depen den ce of th e ch em ical sh ifts on variable substitu-
en ts9,19,20
.
It was sh own 16,21,22 th at th e sm all ran ge of ch em ical sh ifts can be ex-
plain ed by a com petitive action of two effects: th e electron ic effect of th e
para substituen t th rough bon ds an d th e ch an ge in rin g curren t an isotropy
con tribution to th e sh ieldin g of th e proxim al Hα h ydrogen atom . Th is
ch an ge is again caused by th e para substituen t (X in Fig. 1) sin ce th e elec-
tron den sity redistribution on th e C(aryl)–N bon d ch an ges th e con tribution
from th e con jugation of th e arom atic π-electron con jugation with N=C(α)
π-electron an d N atom n -electron s. Th is affects th e twist an gle aroun d th e
C(aryl)–N bon d an d, con sequen tly, th e position of th e Hα atom with respect
to th e an ilin e rin g.
Hα
HmZ
NO2
X
N
p
HmE
FIG. 1
4-Substituted N-(4-n itroben zyliden e)an ilin es
Collect. Czech. Chem. Commun. (Vol. 69) (2004)

1568
Proks, Holík:
An altern ative explan ation based on th e lack of th e substituen t effect19
due to 90° twist in th e m olecule can be rejected on th e basis of oth er suc-
cessful correlation s, e.g., in tern al ch em ical sh ift of eth yl group16 13C NMR
,
ch em ical sh ift23, an d 19F NMR ch em ical sh ifts24 in para position of th e ben -
1
zyliden e rin g or JCH in azom eth in e C(α)–Hα group22,25 with th e Ham m ett
σp of th e variable para substituen t X in an ilin e rin g. Th ese observation s
sh ow th at th e effect of th e X-substituen t is tran sm itted th rough out th e
wh ole m olecule.
Sin ce th ese successful correlation s can be regarded on ly as an in direct
proof of th e twist in th e m olecule an d of th e rin g curren t con tribution to
th e ch em ical sh ieldin g, we h ave decided to rein vestigate th is topic usin g ab
in itio calculation s26. Our aim was to prove th at th e dih edral an gle between
th e an ilin e rin g an d C(aryl)–N(=)–Hα plan e depen ds on th e X-substituen t
1
an d th at th e H NMR ch em ical sh ifts of Hα corrected for rin g curren t sh ield-
in g correlate well with th e electron ic effect of th e X-substituen t. In particu-
lar, we evaluated th e rin g curren t ch em ical sh ieldin g by direct ab in itio cal-
culation 26 in th e poin t of in terest aroun d th e m odel system .
Of differen t variables ch aracterizin g th e effect of th e substituen t in th e
1
para position we h ave selected H NMR ch em ical sh ift in crem en ts18, ap, on
th e basis of th e h igh est coefficien ts of correlation to in vestigated vari-
ables27. Neith er 13C NMR ch em ical sh ift in crem en ts18, zp, n or Ham m ett σp
or dual σF an d σR con stan ts describe better th e tran sm ission of th e X-
substituen t effect to th e para position of th e an ilin e rin g, to th e N=CH
group is con cern ed.
EXPERIMENTAL
N-Ben zyliden ean ilin es were prepared by m eltin g equim olar am oun ts of a 4-substituted an i-
lin e an d 4-n itroben zaldeh yde in san d bath un til th e evolution of water vapor ceased. Th is
took at 120 °C typically 30 m in . Th e products were crystallized from eth an ol. Th e ben zyl-
iden ean ilin es were ch aracterized by th eir m eltin g poin ts (Table I). All com poun ds gave satis-
factory elem en tal an alyses an d were pure accordin g to TLC on silica. 1H NMR spectra were
recorded on a 500 MHz Bruker DRX AVANCE spectrom eter in dim eth yl sulfoxide (DMSO-d6)
un der con dition s of in fin ite dilution usin g tetram eth ylsilan e (TMS) as an in tern al stan dard.
Orth o an d m eta ch em ical sh ifts for p-substituted ben zen es were calculated as for an AA′BB′
system .
Quan tum ch em ical calculation s were perform ed on an SGI Power Ch allen ge XL super-
com puter. Geom etries of 4-substituted N-(4-n itroben zyliden e)an ilin es studied h ere were op-
tim ized usin g differen t quan tum ch em ical m eth ods. First, at th e HF/6-31G* level, th e above-
m en tion ed dih edral an gle was set to values from th e in terval between 0 an d 180° with th e
in crem en t of 45° an d th e full geom etrical optim ization was perform ed. For each m olecule
th is procedure con verged to th e sam e con form er. Subsequen tly, th e geom etries of th ese con -
Collect. Czech. Chem. Commun. (Vol. 69) (2004)

4-Substituted N-(4-Nitrobenzylidene)anilines
1569
form ers were em ployed as an in itial guess for full geom etrical optim ization s usin g
B3LYP/6-311G** approach . Moreover, th e h arm on ic vibration al frequen cies for th e
B3LYP/6-311G** geom etries of all th e com poun ds were calculated at th is level of th eory; all
th e frequen cies were foun d to be real. Con sequen tly, it was th us verified th at m in im a were
obtain ed. Th e den sity fun ction al th eory based B3LYP approach com bin es Becke’s th ree-
param eter exch an ge28 an d th e Lee–Yan g–Parr correlation 29 fun ction als. For NMR sh ieldin g
calculation th e gauge-in cludin g atom ic orbital (GIAO) form alism an d th e B3LYP/6-311G**
m eth od were adopted. Th is com putation al strategy also provides for th e curren t den sities
(Fig. 2 sh ows an exam ple).
All quan tum ch em istry calculation s were perform ed usin g th e Gaussian 94 suite of pro-
gram s26, statistical calculation s were carried out on PC usin g own scripts written in Matlab
(Th e Math Works, Natick (MA), U.S.A.)
RESULTS AND DISCUSSION
Sin ce th e success of correlation stron gly depen ds on th e n um ber of m ea-
surem en ts (in th e presen t case equal to th e n um ber of com poun ds), fifteen
4-substituted N-(4-n itroben zyliden e)an ilin es for substituen ts (see Fig. 1 an d
Table I) were prepared. As was observed previously16, th e correlation of th e
ch em ical sh ift of th e Ho h ydrogen δ(Ho) with in crem en ts of ch em ical sh ift
ao is satisfactory (cf. Eq. (1); r is th e correlation coeficien t an d se is th e stan -
dard error of th e δ(Ho).
δ(Ho) = 0.90(0.08) ao + 7.41(0.04)
r = 0.9514, se = 0.1603
(1)
FIG. 2
Th e electron den sity distribution of 4-h ydroxy-N-(4-n itroben zyliden e)an ilin e obtain ed from
th e y-com pon en t of th e B3LYP-GIAO/6-311G**-com puted curren t den sity (th e y-axis is di-
rected approxim ately parallel to th e plan e of th e an ilin e rin g)
Collect. Czech. Chem. Commun. (Vol. 69) (2004)

1570
Proks, Holík:
In con trary to th at, th e correlation of th e Hα proton with th e ap ch em ical
sh ift in crem en ts is un reliable. Nam ely, th e regression slope approach es zero
wh ich could be erron eously taken as an in depen den ce of δ(Hα) on ap in cre-
m en ts of ch em ical sh ift (see Eq. (2) an d discussion above).
δ(Hα) = 0.06(0.02) ap + 8.806(0.006)
r = 0.6677, se = 0.02
(2)
Low se value (0.02) testifies th at som e degree of correlation between th e
ch em ical sh ifts an d th e ap in crem en ts exists. Moreover, th e correlation co-
efficien t of orth ogon al regression 30 – th e value n ot depen den t on th e slope
of th e regression lin e – is h igh (0.9981) due to a close proxim ity of th e
poin ts to th e regression lin e.
TABLE I
Ch aracteristics of th e prepared 4-substituted N-(4-n itroben zyliden e)an ilin es (in crem en ts of
ch em ical sh ift ao, am , an d ap; 1H NMR ch em ical sh ift of Hα, Ho, an d Hm ; th e experim en tal
an d publish ed (referen ces are given in paren th eses) m eltin g poin ts)
δ(Hα)
ppm
δ(H0)
ppm
δ(Hm
ppm
)
M.p.
°C
M.p.
°C (lit.)
a0
am
ap
En try
X
1
2
NEt2
NH2
OMe
OH
–0.66
–0.75
–0.48
–0.56
0.12
–0.18
–0.25
–0.09
–0.12
–0.07
0.00
–0.67
–0.65
–0.44
–0.45
–0.28
–0.20
–0.09
–0.04
–0.17
0.00
8.7961
8.7721
8.8065
8.7246
8.7817
8.7946
8.7955
8.7918
8.8047
8.8048
8.7987
8.8216
8.8220
8.8365
8.8400
6.7153
6.6233
7.0059
6.8429
7.6585
7.2689
7.4854
7.6128
7.4127
7.4548
7.9095
8.0336
8.0319
7.4917
8.3095
7.3603
7.2435
7.3881
7.2860
7.3394
7.4127
7.3660
7.2962
7.2737
7.3475
7.4574
7.4112
7.4110
7.3781
7.5101
142–143 146 (34)
166–167 NAa
3
134–135 136–139 (35)
4
168
168.5 (36)
5
NHCOMe
F
223–226 223–226 (37)
112–113 109–110 (38)
6
–0.26
0.03
7
Cl
–0.02
–0.08
–0.06
0.00
134
134–136 (35)
8
Br
0.18
160–161 166–167 (37)
122–123 125–127 (35)
9
Me
–0.14
0.00
10
11
12
13
14
15
H
89–90
190
89–90.5 (39)
172 (37)
CN
0.36
0.18
0.28
COMe
COOEt
CONH2
NO2
0.62
0.14
0.21
143–144 146 (40)
177–178 177–178 (38)
187–188 NAa
0.71
0.11
0.21
0.61
0.10
0.17
0.95
0.26
0.38
202–203 200 (41)
a
NA, n ot available.
Collect. Czech. Chem. Commun. (Vol. 69) (2004)

4-Substituted N-(4-Nitrobenzylidene)anilines
1571
Clearly, th ere are several differen t con tribution s to th e ch em ical sh ift
of Hα in th e in vestigated system s. In 1997 Wawer et al.15 prepared n in e
4-substituted N-[4-(dim eth ylam in o)ben zyliden e]an ilin es an d calculated by
th e sem iem pirical AM1 m eth od38 th eir geom etries. Dih edral an gles be-
tween th e an ilin e rin g an d th e N=C–Hα plan e were foun d in th e ran ge from
28° (X = OMe) to 35.6° (X = NO2). Th e correlation with ap in crem en ts was
rath er good (r = 0.9650; our result).
Th e resultin g dih edral an gles are sh own in Table II an d regression param -
eters with ap variable are in th e upper part of Table III. It can be seen th at
all param eters are statistically sign ifican t – th eir ratio to correspon din g
stan dard deviation (sb0 an d sb1, respectively) is always larger th an th e criti-
cal value of Studen t’s t-distribution .
Th e n ext step was th e determ in ation of th e an isotropy con tribution due
to th e an ilin e rin g curren t to th e sh ieldin g of th e Hα atom . Th ree differen t
TABLE II
Dih edral an gles (in °) between an ilin e rin g an d th e rest of th e m olecule in 4-substituted N-
(4-n itroben zyliden e)an ilin e series as obtain ed by th e DFT geom etrical optim ization s, values
of th e ben zen e rin g curren t an isotropy con tribution ∆σ(Hα) to th e ch em ical sh ift of Hα th e
absolute ch em ical sh ieldin g in HN=CH2 m odel system at th e position s correspon din g to th e
H
m , an d th e average value ∆σ(Hm ). See th e text for details
∆σ(Hα)
ppm
∆σ(Hm Z
ppm
)
∆σ(Hm E
ppm
)
∆σ(Hm )
ppm
En try
Substituen t
γ
1
2
NEt2
NH2
OCH3
OH
21.272
26.828
30.256
30.568
33.290
36.812
37.799
40.170
36.986
39.550
42.117
43.179
42.178
40.321
45.409
–0.4984
–0.4855
–0.4553
–0.4472
–0.4150
–0.3804
–0.3488
–0.3435
–0.3770
–0.3400
–0.2919
–0.2974
–0.3134
–0.3303
–0.2759
–0.1032
–0.0918
–0.0716
–0.0655
–0.0418
–0.0160
–0.0430
0.0063
–0.0133
0.0106
0.0468
0.0456
0.0317
0.0180
0.0606
–0.4827
–0.4594
–0.4271
–0.4159
–0.3771
–0.3362
–0.3084
–0.3040
–0.3306
–0.2952
–0.2448
–0.2485
–0.2674
–0.2988
–0.2263
–0.2929
–0.2756
–0.2494
–0.2407
–0.2095
–0.1761
–0.1757
–0.1489
–0.1720
–0.1423
–0.0991
–0.1015
–0.1179
–0.1404
–0.0828
3
4
5
NHCOCH3
F
6
7
Cl
8
Br
9
CH3
10
11
12
13
14
15
H
CN
COCH3
COOEt
CONH2
NO2
Collect. Czech. Chem. Commun. (Vol. 69) (2004)

1572
Proks, Holík:
strategies for th e determ in ation of th e rin g curren t con tribution to th e
sh ieldin g of azom eth in e h ydrogen Hα were used, n am ely, th e McCon n ell31
approach , Haigh –Mallion 32 m odel, an d direct ab in itio calculation of th e
ch em ical sh ieldin g in a m odel system . However, th e sem iem pirical m eth -
ods by McCon n ell an d Haigh –Mallion did n ot yield satisfactory results
(data n ot sh own ). In th e ab in itio approach , th e absolute ch em ical sh ield-
in g (∆σ) was calculated for th e position in space correspon din g to th e posi-
tion of Hα in th e com poun ds in vestigated. Th us, th e system was approxi-
m ated by ben zen e an d th e ch em ical sh ieldin g was calculated at th e poin t
in space origin ally occupied by th e Hα h ydrogen of a given N-(4-n itro-
ben zyliden e)an ilin e. Th e B3LYP/6-311G** geom etries were used. Th e ch em -
ical sh ieldin g data th us obtain ed can be used as an approxim ation of th e
rin g-curren t con tribution of th e ben zen e rin g to Hα h ydrogen . Resultin g
∆σ(Hα) values are given in Table II. Th ese calculated values of th e diam ag-
n etic an isotropy depen d on th e dih edral an gle.
∆σ(Hα) = 0.0104(0.0006) γ – 0.75(0.02)
r = 0.9809, se = 0.0144
(3)
Moreover, th e diam agn etic an isotropy data can be successfully correlated
with in crem en ts of ch em ical sh ift ap (see Table III).
TABLE III
Results of lin ear regression s between a given depen den t variable an d th e ap ch em ical sh ift
in crem en ts; sym bols b0, b1, r, an d se stan d for th e in tercept, slope, correlation coefficien t,
an d stan dard error of estim ate, respectively. In th e upper part th e regression s of dih edral an -
gles vs ap in crem en ts are given , in th e lower part th e results accordin g to Eq. (4) are sum m a-
rized (see th e text for details)a
Num ber
of th e
outlier
Depen den t
variable
b0
sb0
b1
sb1
se
r
γ
38.7140
–0.3487
8.8059
8.4572
8.4601
0.4891
0.0030
0.0059
0.0058
0.0033
19.5259
0.2125
0.0558
0.2682
0.2536
1.4259
0.0088
0.0172
0.0168
0.0099
0.9671
0.9888
0.6677
0.9754
0.9910
1.1858
0.0111
0.0216
0.0210
0.0119
1
NAb
∆σ
Hα
4
Hα + ∆σ
Hα + ∆σ (–4)
4
11
a
γ values from Table I; ∆σ values from Table II; (–4) = poin t 4 left out from th e regression .
NA, n ot available.
b
Collect. Czech. Chem. Commun. (Vol. 69) (2004)

4-Substituted N-(4-Nitrobenzylidene)anilines
1573
After addition of th e calculated sh ieldin g ∆σ(Hα) (Table II) to th e experi-
m en tal ch em ical sh ifts (i.e., subtraction of th e con tribution to th e sh ield-
in g), th e ch em ical sh ifts free of th e ben zen e rin g curren t con tribution were
obtain ed. Th ese sh ift data were correlated with in crem en ts of ch em ical sh ift
ap accordin g to Eq. (4).
δ(Hα) + ∆σ = b0 + b1 *ap + ε ,
(4)
wh ere ε is th e residual.
Th e correlation s were m uch better th an in th e case with out th e correc-
tion for diam agn etic an isotropy con tribution (Table III, Hα row).
In all th ese regression s th e poin t 4 (X = OH) was classified as an outlier27
.
Th is can be caused by solven t effect: our spectra were m easured in
DMSO-d6, wh ile th e in crem en ts18 of ch em ical sh ifts, ap, were obtain ed from
1H NMR experim en ts in CDCl3. Probably, th e substituen t group OH in ter-
acts with solven t m ore stron gly th an th e oth ers, th erefore, th e ap value for
OH group is biased. Wh en th is poin t was left out (last row in Table III), th e
value of correlation coefficien t sign ifican tly in creased. An oth er con tribu-
tion related to th e solven t effect is caused by th e absen ce of solven t m ole-
cules in th e perform ed quan tum ch em ical calculation s. In particular, an in -
clusion of explicit solven t m olecules in th e calculation leads to a sm all
ch an ge of th e dih edral an gle γ as com pared with in vacuo results33. Th e
quality of correlation s is also affected by th e fact th at all m odels em ployed
are restricted to ben zen e m olecule an d do n ot explicitly con sider th e effect
of th e X-substituen t on th e electron den sity in th e rin g. Un derstan din g
th e relation sh ip between th e degree of con jugation , dih edral an gle an d
X-substituen t can be furth er used in a study of solven t-depen den t ben zyl-
iden ean ilin e geom etries, wh ich in progress33.
Sim ilarly to th e case of Hα h ydrogen , correlation of Hm with in crem en ts
of ch em ical sh ift is n ot optim al (Eq. (5)).
δ(Hm ) = 0.43(0.08) am + 7.37(0.01)
r = 0.8452, se = 0.04
(5)
Th us, in th e sam e way as in th e case of Hα proton , an attem pt was m ade
to estim ate th e com bin ed in fluen ce of diam agn etic an isotropies of C=N
double bon d an d of th e lon e pair of n itrogen to th e ch em ical sh ifts of Hm ’s.
Subsequen tly, th e Hm proton ch em ical sh ifts free of th is com bin ed con tri-
bution were correlated with am in crem en ts of ch em ical sh ift for m on o-
Collect. Czech. Chem. Commun. (Vol. 69) (2004)

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Proks, Holík:
substituted ben zen es. Nam ely, th e diam agn etic an isotropy con tribution s
were com puted for th e m odel system H2C=NH in th e sam e geom etry as ob-
tain ed from th e B3LYP/6-311G** optim ization s of th e com poun ds from our
set. Th us, th e absolute ch em ical sh ieldin g in th e position s correspon din g to
Hm proton s was calculated by th e sam e B3LYP-GIAO approach as in th e
case of Hα (vide supra). As was th e case with Hα proton s, th e McCon n ell
an d Haigh –Mallion m odels were n ot able to provide for satisfactory correla-
tion s (data n ot sh own ). Clearly, th e HZ an d HE proton s differ in th eir posi-
tion with respect to th e C=N bon d. As a con sequen ce, th e absolute ch em i-
cal sh ieldin g at th e correspon din g poin ts aroun d th e H2C=NH m odel is
m arkedly differen t (cf. Table II). However, due to a practically free rotation
aroun d th e C–N bon d (for exam ple, th e B3LYP/6-311G**-calculated rota-
tion barrier for 4-n itro-N-(4-n itroben zyliden e)an ilin e is 1.302 kcal/m ol 33),
th e average of th e ∆σ(Hm Z) an d ∆σ(Hm E) values can be taken as th e ch em ical
sh ieldin g of th e ∆σ(Hm ). Th e resultin g correlation is sum m arized by Eq. (6).
δ(Hm ) + ∆σ(Hm ) = 0.87(0.05) am + 7.195(0.007)
r = 0.9770, se = 0.03
(6)
Th e correlation is n ow m arkedly im proved wh en com pared to th e results
obtain ed for data wh ich are n ot corrected for th e effect of diam agn etic an i-
sotropy.
CONCLUSIONS
By usin g quan tum ch em ical calculation s of th e 4-substituted N-(4-n itro-
ben zyliden e)an ilin es, th e twist of an ilin e rin g, γ, from th e rest of th e m ole-
cule was sh own . Th e ability to predict γ values could be em ployed in m od-
elin g work on th e properties of polyazom eth in es. Both frequen tly used
m odels of th e sh ieldin g exerted by th e arom atic rin g curren t, i.e.,
McCon n ell’s an d Haigh –Mallion ’s, were foun d in adequate for th is system ;
on ly th e ab in itio calculated ch em ical sh ieldin g based on th e geom etries
optim ized by th e DFT B3LYP/6-311G** m eth od was foun d satisfactory for
lin ear regression s between th e variable electron ic effect of X-substituen t
an d th e ch em ical sh ift in crem en ts.
Time allocation in the Czech Academic Supercomputer Center is gratefully acknowledged.
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