E. D. Raczyn´ska et al.
This change may be explained by cross n–π conjugation present
in guanidines and absent in formamidines (Scheme 1). The cross
n–π conjugationaffectstheMullikenatomiccharges, theCNbond
lengths, and consequently, the bond strength and fragmentation.
Linear relationships between the logarithm of ion abundances
ratio and substituent constants are observed for the favored
fragments. For TEPG, the preferential routes of fragmentation are
slightly different than those for TMAGs. Larger alkyl groups at
the N atom increase electronic effects and change the stability
fragments. Although the [M–NEt2]+ ion is still important, the
[C7H16N3]+ fragment is favored.
+
+ [32]
) and selected
Table 6. Substituent constants (σp and σR
spectral data for TMAGs
log{I[M–H]+ log{I[M–Me]+ log{I[M–NMe2]+
+
+
Z
σp
σR
/I[M]+•
}
/I[M]+•
}
/I[M]+•
}
4-Cl
4-F
H
0.11 −0.17
−0.07 −0.25
0.00 0.00
−0.69
−0.64
−0.85
−0.72
−0.70
−0.04
−0.07
−0.11
−0.12
−0.06
0.27
0.21
0.19
4-Me −0.31 −0.08
4-OMe −0.78 −0.42
0.08
−0.12
Acknowledgements
.
+
+
-
-
log{I[M-NMe2] /I[M]
}
0.5
0.0
DFT calculations were conducted using the resources of 45-
processor Beowulf cluster at the Faculty of Chemistry (University of
Gdan´sk) and the Informatics Center of the Metropolitan Academic
Network (IC MAN, Gdan´sk).
References
+
σp
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Chichester, 1991.
-0.5
0.5
-0.5
log{I[M-NMe2]+/I[M]+
0.0
-
-
.
}
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0.0
ˇ
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+
σR
0.0
-0.5
-0.4
-0.2
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Figure 8. Relations between log{I[M–NMe2]+/I[M]+•} of TMAGs and
substituent constants.[32]
.
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+
+
(Table 6). Linear relationships with σp (r = 0.161) and σR
(r = 0.659) constants are not significant for TMAGs. Similarly,
substituent Z at the phenyl ring has no significant effect on
the log{I[M–Me]+/I[M]+•} values which vary solely from −0.04
for Cl to −0.12 for Me. Quite a different situation takes place
for the favored route of fragmentation, i.e. loss of the NMe2
[14] E. D. Raczyn´ska, M. Decouzon, J.-F. Gal, P.-C. Maria, R. W. Taft. J.Mass
Spectrom. 1998, 33, 1029.
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+
+
group.Thelog{I[M–NMe2]+/I[M]+•}valuescorrelatewellwithσp
constants (Fig. 8): log{I[M–NMe2]+/I[M]+•} = 0.22 + 0.43 · σp
[17] R. W. Taft,
E. D. Raczyn´ska,
P.-C. Maria,
I. Leito,
J.-F. Gal,
+
with r = 0.993 (n = 5). Relation with σR is not linear. This
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41.
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NMe2 radical at the C functional atom (separated by the N atom
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effect. Unfortunately, five points are not sufficient to perform
valuable multiparameter analysis of substituent effects, in order to
separate the resonance and field/inductive contributions.
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Conclusions
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Investigations of electron-ionization mass spectra for analogous
series of DMAFs and TMAGs showed an interesting change of
the preferential routes of fragmentation from favored formation
of the [M–H]+ benzimidazolium ions for DMAFs to favored
formation of the [M–NMe2]+ benzisonitrilium ions for TMAGs.
c
Copyright ꢀ 2010 John Wiley & Sons, Ltd.
J. Mass. Spectrom. 2010, 45, 762–771