Determination of heats of tautomerization of nitrile-ketenimine by mass spectrometry

Article abstract of DOI:10.1255/ejms.1112

Tautomerism of some nitriles has been studied by mass spectrometry. The analysis of the corresponding mass spectra has allowed some fragmentations to specific tautomers to be assigned and the heats of tautomerization to be determined through temperature effects and electron energy studies. Experimental determinations are supported by theoretical calculations. The joint analysis of mass spectrometry and DFT-B3LYP data indicate that this tautomeric equilibrium can be studied by the experimental spectrometric strategy employed.

Full text of DOI:10.1255/ejms.1112

H. Saraví et al., Eur. J. Mass Spectrom. 17, 125–143 (2011)
125
received: 7 January 2011
n
revised: 8 March 2011 n Accepted: 10 March 2011 n Publication: 24 March 2011
EuroPEAN
JourNAL
of
MASS
SPEctroMEtry
Determination of heats of tautomerization of
nitrile–ketenimine by mass spectrometry
Hebe Saraví Cisneros,a Mauricio F. Erben,b Carlos O. Della Védova,b,c Sergio Laurella,a Patricia E. Allegrettia,*
and Jorge J.P. Furlonga,*
aLADEcor (uNLP), División Química orgánica, Departamento de Química, facultad de ciencias Exactas, universidad Nacional de La Plata,
calle 47 y 115, (1900) La Plata, república Argentina. E-mails: furlong@quimica.unlp.edu.ar and pallegre@quimica.unlp.edu.ar
bcEQuINor (uNLP–coNIcEt, cct La Plata), Departamento de Química, facultad de ciencias Exactas, universidad Nacional de La Plata. cc
962, (1900) La Plata, república Argentina
cLaboratorio de Servicios a la Industria y al Sistema científico, LaSeISic (uNLP-cIc-coNIcEt), camino centenario e/505 y 508, (1903) Gonnet,
república Argentina
Tautomerism of some nitriles has been studied by mass spectrometry. The analysis of the corresponding mass spectra has allowed
some fragmentations to specific tautomers to be assigned and the heats of tautomerization to be determined through temperature
effects and electron energy studies. Experimental determinations are supported by theoretical calculations. The joint analysis of mass
spectrometry and DFT-B3LYP data indicate that this tautomeric equilibrium can be studied by the experimental spectrometric strategy
employed.
Keywords: heats of tautomerization, nitriles, mass spectrometry, quantum chemical calculations
Introduction
A few studies where enolization of nitriles is proposed to take
place have been found in the recent literature.1–3 Most of nitriles
in polar hydroxylic solvents) was assigned to the ocurrence
of a ketenimine structure.3 contrarily, the long wavelength
appear to strongly favor the cyano form in this equilibrium. uV absorption band observed in the spectra of some alkyli-
Kasturi et al. have carried out the study of the ultraviolet (uV
absorption spectra of a number of 1,2-dicyano esters and
1,1,2-tricyano compounds demonstrating the presence of
nitrile–ketenimine tautomerism.2 Among these tautomeric
compounds, those which involve a methylene hydrogen g to
the nitrile group and electron withdrawing groups such as cN
or coor (Scheme 1)3 are of particular interest.
dene malononitriles and cyanoacetates has been claimed to
be a consequence of the anion formation and not of nitrile–
ketenimine tautomerism.4
In addition, absorption bands between 2100 cm−1 and
1500 cm−1 in the infrared (Ir) spectra of these compounds
(which could be expected if any ketenimine had been present)
were not observed.5 on the other hand, the use of a highly
enantioselective direct dialkyl allylic electrophilic functionali-
In connection with the synthesis of heterocyclic-containing
compounds, Kasturi et al. have synthesized several conden- zation by the addition of diethyl azodicarboxylates to alkylidene
sation products of b-ketoesters with malonitrile and ethyl- cyanoacetates and malononitriles has been demonstrated and
cyanoacetate. from the analysis of the uV spectra in ethanol
and in ethanol/sodium hydroxide solution, the observed hyper-
chromic effect on the band around 355nm (detectable only
can be applied to other electrophilic addition reactions.6
Additional experimental evidence can be taken into account
considering that some reactions can only take place through
ISSN: 1469-0667
doi: 10.1255/ejms.1112
© IM Publications LLP 2011
All rights reserved

126
Determination of Heats of Tautomerization of Nitrile–Ketenimine
Scheme 1.
a specific tautomer (ketenimine). Amination was selected and, for fragments assigned specifically to both tautomers also
although a mechanistic study of amination of ketenimines
is missing, it is known that amination of ketenimines forms
amidines.7 High level ab initio calculations conducted by Sung
et al. concluded that amination of ketenimines proceeds via
vary with small changes in the ion source temperature which
would indicate that tautomerism in this case is unimolecular.
the simplest interpretation takes into account the pressure at
different instrument points: the injection system where inter-
amine addition across the c=N bond rather than the c=c bond, molecular collisions can occur (tautomerization is usually
followed by tautomerization to form the amidine product. An
intermediate with vinylidendiamine structure has been directly
observed by low-temperature 1H nuclear magnetic resonance
(NMr) spectroscopy.8
bimolecular) and the ion source where only unimolecular
processes can take place (an intramolecular mechanism on
the low pressure side could also explain those results).
Although NMr is particularly useful in the determination of
equilibrium constants, this methodology not always provides
the expected data for several reasons (very low concentration
Mass spectrometry has already been demonstrated as a
useful tool for the study of tautomerism.9–23 this is the reason
for the selection of this methodology to study the nitrile– of tautomers can be one of them). Mass spectrometry repre-
ketenimine equilibrium trying to find experimental evidence of sents a versatile tool for the evaluation of thermodynamic
the occurrence of the ketenimine tautomer through the inter- data on gas-phase ions.26 this methodology has been already
pretation of the mass spectral peaks of the selected nitriles.
the study of temperature effects (injector vs ion source) on
the tautomerism of selected compounds has demonstrated
that equilibration occurs inside the injection system prior
to ionization, which might be considered as evidence of the
lack of contribution to the mass spectral data (used to eval-
uate these equilibria) by tautomerization of radical ions. In
fact, it has been claimed that tautomerization also occurs
in the molecular ions, although some reports proposed that
used for the calculation of heats of tautomerization of selected
thioamides and measurably reliable results have been found.
Very good correlations with theoretical data for such thermo-
dynamic property have given support to this approach.20
Experimental
Synthesis of nitriles
enolization should be considered to occur before ionization, the 2-sec-butylidenmalononitrile (I), 2-cyano-3-methylpentan-
since no evidence of tautomerism of ionc species could be
2-oic acid (II) and 4,4-dicyano-3-methyl-3-butenoic methyl
ester (III) were synthesized according to the condensation
procedure of cope–Knoevenagel (Scheme 2).27–31
observed.10,24
from b-diketones, the analysis of their mass spectra
demonstrated that fragmentation patterns are influenced
All compounds were characterized by spectrometric
by the keto–enol content of these compounds. this simpli- methods including Ir, MS and NMr techniques.
fied analysis might have the disadvantage that the ion abun-
Gas chromatography–mass spectrometry–
dances may not only depend on tautomerization but also
among other parameters, on bond strength differences.
Notwithstanding it has been demonstrated not to be the case
single quadrupole
these measurements were recorded on a GcMS-QP2010
for some compound families. Based on temperature effects, SHIMADZu instrument using helium as the carrier phase
Zamir and co-workers24 reported for selected b-diketones that
(column head pressure 100 kPa). the column used was a
a change in injection temperature results in changes of rela- HP-5 (30 m×0.32mm×0.25mm film thickness). A 1µL injec-
tive peak intensities for fragments assigned to the enol form, tion of the compounds dissolved in acetone was chromato-
while changes in the ion source temperature did not exert any
effect.
graphed under the following conditions: injector tempera-
tures were 250°c, 275°c and 300°c. the column temperature
In a similar study, but for open–closed chain tautomeriza- program was as follows: 70°c (2min), 7°cmin–1, 200°c (2min),
tion, cooks et al.25 observed that the ion abundance ratios
5°cmin–1, 300°c (2min). In the spectrometer the ion source

H. Saraví et al., Eur. J. Mass Spectrom. 17, 125–143 (2011)
127
Scheme 2.
was kept at 230°c for most determinations. temperature
effects at the ion source were assayed at 185°c. the electron
energies were 70eV, 50eV and 30eV. the pressure in the mass
spectrometer was lower than 10−5 torr, thus precluding ion
molecule reactions.
(s, rcH2), 1378 (s, nc–cN), 1071 (m, nc–cH2), 877 (w, rcH2),
794 (w, rcH3).
2-cyano-3-methylpentan-2-oic acid (II): 3474 (s, broad, no–H),
2986 (s, nc–H), 2943 (m, nc–H), 2224 (s, ncºN), 1718 (vs,
nc=o), 1602 (vs, nc=c), 1434 (vs, rcH2),1374 (s, nc–o), 1291 (vs,
rcH2), 1260 (vs, ro–H), 1224 (m, nc–cN), 1101 (w, rcH2), 1066
(m, rcH3), 925 (m, rcH2), 799 (w, rcH3) , 742 (m, rcooH).
4,4-dicyano-3-methyl-3-butenoicmethylester(III):2958(s,nc–H),
2236(s,ncºN),1744(vs,nc=o),1608(vs,nc=c),1435(vs,rcH2)1375
(vs, nc–c(o)), 1332 (vs, rcH2), 1250 (m, nc–cN), 1195 (s, nc(o)–
o), 1177 (s, nH3c–o), 1070 (m, nc–cH2), 1024 (w, rcH3), 998 (m,
Gas chromatography–mass
spectrometry–ion trap
these determinations were carried out by injection of solutions
of compounds I–III in acetone (1µL) in a thermo Quest trace
2000 chromatograph coupled to a finnigan Polaris ion trap
detector (unit mass resolution) under the same temperature, rcH2), 862 (w, rcc(o)c), 766 (w, roco).
pressure and electron energy conditions mentioned for the
Quantum chemical calculations
quadrupole system. the ion trap was utilized to confirm the
proposed fragmentation pathways by collision-induced disso- the calculations were performed using the GAuSSIAN 03
ciation (cID) using helium as the damping gas, a cID voltage of
4–8eV and an excitation energy of 0.3–0.5 (optimized for each
transition). tandem mass spectrometry (MS/MS) full scan ion
product spectra were recorded for selected precursor ions.
program package. optimum equilibrium geometries were
computed for the singlet ground states of all pertinent molec-
ular systems using Dft-B3LyP with the 6-31G* and the more
extended 6-311++G**(d,p) basis sets. Numerous conforma-
tions were computed in order to ensure that the lowest energy
conformation was obtained for each molecular system. for
each conformation, harmonic vibrational frequencies were
also calculated at the same level of computation to guarantee
that each optimized geometry corresponds to a true local
minimum and obtain the zero-point energy correction (ZPE).
Vibrational spectroscopy
Ir spectra of the liquid 2-sec-butylidenmalononitrile and
both solid 2-cyano-3-methylpentan-2-oic acid and solid
4,4-dicyano-3-methyl-3-butenoic methyl ester in KBr pellets)
were recorded with a resolution of 2cm−1 in the 4000–400cm−1
range on a Bruker EQuINoX 55 ftIr spectrometer.
Signal assignments (cm−1) are given for the three compounds
under investigation:
Error calculations32,33
When simple linear correlations and graphical methods are
2-sec-butilydenmalononitrile (I): 2983 (s, nc–H), 2944 (m, used, the experimental error can be calculated by linear regres-
nc–H), 2883 (m, nc–H), 2232 (vs, ncºN), 1600 (vs, nc=c), 1463 sion (minimum square method). In this work thermodynamic

128
Determination of Heats of Tautomerization of Nitrile–Ketenimine
data are calculated from the slope of linear plots, so that this
approach was chosen and the errors in table 3 calculated
compounds9–23 that there is no significant interconversion
of the tautomeric forms in the gas phase following electron
accordingly. the standard error for the slops of the linear plots, impact ionization in the mass spectrometer (molecular ions,
(S(b)), is calculated by a Microsoft Excel program using the
following equations:
M+• do not seem to undergo unimolecular tautomerization)
and, even more surprising, for gas chromatography (Gc)/MS
experiments, once the solvent is separated after injection in
the injection port of the gas chromatograph, tautomerism
mechanisms (intermolecular, unimolecular) would not seem
to take place, even with no Gc separation (under the selected
experimental conditions). these conclusions are supported by
temperature studies at the ion source (negligible effect) and
at the injection port of the gas chromatograph with a shifting
effect in agreement with the corresponding heats of tautomer-
ization.20 In fact, this process would take place very fast under
the working conditions in the Gc.
½
S(b) =S(y)/S(x,x)
½
S(y) ={[Si(yi −ya)2]/(n−1)}
and
2
S(x,x) =Sxi2 −nxa
where n is the number of data points, xi and yi are the plotted
data and xa and ya the corresponding averaged values.
Separation of tautomers in the analytical column is
frequently very difficult, consequently the different pathways
of fragmentation of the tautomeric forms have to be used
for identification of individual tautomers.20 for this reason
and because of the high similarity between MS (commercial
databases) and Gc/MS spectra, analytical separation has not
Results and discussion
Mass spectrometry
the relevance of spectrometric data as a predictive tool
in regard to tautomeric equilibria depends mainly on the
fact that the contribution due to tautomerization of molec- been considered critical for the present work. Analogously,
ular ions in the gas phase does not take place or can be it is thought that most of the conclusions could be useful to
ignored. the importance of this point comes from the physi- analyze spectra registered with mass spectrometers equipped
cochemical properties of ionic and radicalary species, quite
different from the neutral ones. this could be the reason for
with direct insertion probes.
the mass spectrum of 2-sec-butilydenmalononitrile (I) at
possible distortion of results and loss of the desirable predic- 70eV is shown in figure 1. the peaks corresponding to m/z
tive power of the methodology. High-energy ionic species
as the molecular ions of the selected nitriles can undergo
1,5-hydrogen shifts (many rearrangements are shown in the
119 (M−H)+, m/z 105 (M−cH3)+, m/z 93 (M−cNH)+., m/z 92 and
m/z 64 can be explained from both tautomers, as shown
in Scheme 3(a) (nitrile form) and Scheme 3(b) (ketenimine
form).
Ionization on the cN triple bond allows, through hybridiza-
tion change, the necessary spatial interaction for the detailed
rearrangements. a-ruptures and inductive cleavages would
be responsible for the corresponding fragment ions. the ion
schemes) although the main tautomer structure seems to be
+
not affected: c=c–cºNH is thermodynamically more stable
+
than c–c=c=NH.
It has been demonstrated in the case of keto–enol
tautomerism of a variety of carbonylic and thiocarbonylic
Figure 1. Mass spectrum of the 2-sec-butilydenmalononitrile (I) at 70eV.

H. Saraví et al., Eur. J. Mass Spectrom. 17, 125–143 (2011)
129
at m/z 105 is stabilized by electron shifts to a ketenimine-like
structure.
stability would become the corresponding driving force for
the fragmentation.
the ion at m/z 91 can also be assigned to the ethyl loss from
for graphical simplicity, neutral species with cumulative
the ketinimine structure by assuming that the initial ioniza- double bonds (rather energy demanding) have been drawn in
tion involves the ketenimine c=c double bond. the ion at m/z
78 can only be explained from the nitrile form as depicted in
Scheme 4 and as a loss of cNH from the fragment ion at m/z
105 in Scheme 3(a).
the fragment ions at m/z 91, m/z 64 and m/z 78 can be
described by a structure with charge migration to account
for the energy involved when cumulative double bonds are
formed.
some of the schemes although cyclization/isomerization might
occur to circumvent relative high energy barriers. In case of
ionic fragments drawn with this structural feature, stability
can be assumed by charge migration as already mentioned.
Although not clear, isomerization to cyclic compounds cannot
be discarded to facilitate the fragmentation process.
It seems clear from the analysis of the main fragment
peaks that ketenimine occurs, since fragment ions exist
the ions at m/z 39, m/z 50, m/z 55 and m/z 66 are assign- which can only be explained through this tautomer. A rela-
able to the ketenimine form, as shown in Scheme 5. Initial
ionization of the ketenimine form would involve the cN
triple bond of the nitrile moiety or the cNH double bond of
the ketenimine. It is interesting that the abundant m/z 66
[isobaric with that in Scheme 3(b)] is a particularly stable
ion. the ion at m/z 55 is formed by inductive cleavage and its
tive estimation of the tautomers occurrence could be the
ratios m/z 50 to m/z 78, and m/z 39 to m/z 78.
the mass spectrum of 2-cyano-3-methylpentan-2-oic acid
(II) at 70eV is shown in figure 2. the peaks at m/z 124, m/z
122 (M−oH)+, m/z 112 (M−cNH)+•, m/z 110, m/z 94 (M−cooH)+,
m/z 93 can be explained through both tautomeric forms
Scheme 3(a).

130
Determination of Heats of Tautomerization of Nitrile–Ketenimine
Scheme 3(b).
Scheme 4.

H. Saraví et al., Eur. J. Mass Spectrom. 17, 125–143 (2011)
131
Scheme 5.
(Scheme 6). In Scheme 6(a) initial ionization of the cN triple
bond is proposed prior to hydrogen rearrangement and simple
of the carboxylic group in Scheme 6(b), in the carbonyl oxygen
of the same group in Scheme 6(c), in the c=c double bond in
a-ruptures and inductive cleavage fragmentation mecha- Scheme 6(d). Scheme 6(e) shows fragmentations that might
nisms; analogously initial ionization occurs in the oH oxygen take place from the ketenimine tautomers and can also be
Figure 2. Mass spectrum of 2-cyano-3-methylpentan-2-oic acid (II) at 70eV.

132
Determination of Heats of Tautomerization of Nitrile–Ketenimine
Scheme 6(a).
Scheme 6(b).

H. Saraví et al., Eur. J. Mass Spectrom. 17, 125–143 (2011)
133
Scheme 6(c).
Scheme 6(d).
explained by the nitrile forms (with the exception of the ion at
m/z 55 that seems to be explained as coming exclusively from
the ketenimine tautomer).
the ions at m/z 112 (M−cNH)+•, m/z 106 (M−cH3–H2o)+, m/z
97, m/z 92 (M−cH2cH3–H2o)+, m/z 78, m/z 66, and m/z 64 can
be justified from the nitrile form [Schemes 6(a)–d)]. A relative
estimation of the tautomers occurrence could be the ratios
m/z 39 to m/z 97 and m/z 68 to m/z 97.
the mass spectrum of 4,4-dicyano-3-methyl-3-butenoic
methyl ester (III) at 70 eV is shown in figure 3. Peaks at
m/z 149 (M−cH3)+, m/z 138 (M−cN)+, m/z 137 (M−cNH)+•,
the peaks at m/z 39, m/z 55 and m/z 68 would be assignable
to the ketenimine form [Scheme 6(e) and Scheme 7]. the ion
at m/z 67 can also be assigned to the nitrile tautomer but with
a different structure (oc–cH–cN•+). Scheme 7(a) starts with
hydrogen rearrangement in the oH oxygen of the carboxyl
group while Scheme 7(b) does so with hydrogen rearrange- m/z 133 (M−ocH3)+ are observed. the fragment ions at m/z
ment in the cNH nitrogen of the ketenimine.
133, m/z 132, m/z 106, m/z 105, m/z 104, m/z 59 and m/z 31,

134
Determination of Heats of Tautomerization of Nitrile–Ketenimine
Scheme 7(a).
Scheme 7(b).
Figure 3. Mass spectrum of 4,4-dicyano-3-methyl-3-butenoic methyl ester (III) at 70eV.

H. Saraví et al., Eur. J. Mass Spectrom. 17, 125–143 (2011)
135
Scheme 8(a).
Scheme 8(b).

136
Determination of Heats of Tautomerization of Nitrile–Ketenimine
Scheme 8(c).
are reasonable from both tautomeric forms, according to
Schemes 8(a)–(d).
migration. the ion at m/z 104 with the ketenimine structure
would fragment to the ion at m/z 39 by a-rupture.
Scheme 8(a) show fragmentations initiated by hydrogen
rearrangement to the nitrogen after ionization of the cN triple
bond. Scheme 8(b) depicts different fragmentation pathways
for the formation of the ion at m/z 138 from both tautomers
(cN loss). Scheme 8(c) explains fragmentations initiated by
hydrogen rearrangement to the carbonyl oxygen of the ester
moiety while Scheme 8(d) does it by hydrogen rearrangement
to the other oxygen atom.
the ions at m/z 66, m/z 50 and m/z 39 can be justified
only through the ketenimine form (Scheme 9) involving
rearrangements to the nitrile nitrogen or ketenimine nitrogen
as shown. In all these cases, stability of the ions is the driving
force for the corresponding fragmentation.
the fragment ion at m/z 122 can be explained from the
nitrile form (Scheme 10). the depicted pathway offers a
likely fragmentation mechanism and its stability is better
explained taking into consideration the charge migration
(not drawn).
As observed before, fragment ions with cumulative double
bonds can also be described by structures that exhibit charge

H. Saraví et al., Eur. J. Mass Spectrom. 17, 125–143 (2011)
137
Scheme 8(d).
A relative estimation of the tautomers occurrence could be
the ratios m/z 39 to m/z 122 and m/z 66 to m/z 122. the proposed
fragmentation pathways in the mass spectra of nitriles I, II and
III were confirmed by MS/MS experiments in an ion trap mass
spectrometric detector. table 1 summarizes this data.
table 2 indicates the relative abundances for selected ion
fragments which were assigned to specific structures, nitrile
and ketenimine, at different injection port temperatures and
electron energies. Negligible effects were found for different
ion source temperatures (at 230°c and 185°c, data not
reported).
the calculated slopes from figures 4–6 can be used
directly to determine the enthalpy changes [Equation (1)].
the decreasing order observed from I to II and III could be
éf ketenimine
rationalized by substituent effects on both tùautomers.
[
]
ketenimine
DH
=ln ë
û
= -
Ln K = ln
+ C (1)
[
]
nitrile
f nitrile
RT
where [f ketenimine] and [f nitrile] are the abundance of the
fragments corresponding to the ketenimine and nitrile forms,
assuming that the concentration ratios are proportional to the
ion fragment abundance ratios.
the correlation between the heats of tautomerization calcu-
lated from the slopes of figures 4–6 and those determined by
the values measured are in agreement with the occurrence
of an endothermic process as expected for the equilibrium
nitrile-ketenimine.

138
Determination of Heats of Tautomerization of Nitrile–Ketenimine
Scheme 9.
Scheme 10.

H. Saraví et al., Eur. J. Mass Spectrom. 17, 125–143 (2011)
139
Table 1. MS/MS data for the selected nitriles.
Compound
I
Precursor ions (m/z)
Relevant product ions (m/z)
119
105
93
92
78
78, 39
50
66
II
124
121
110
93
106, 97, 78
93, 67, 39
92, 64
39
III
137
133
132
66
122, 106, 64
105
104, 39
50, 39
Table 2. Mass spectral data of nitriles at different sample introduction system temperatures and electron energies.a
70eV
50eV
30eV
Compound
T1
T2
T3
T1
T2
T3
T1
T2
T3
(300ºC)
(275ºC)
(250ºC)
(300ºC)
(275ºC)
(250ºC)
(300ºC)
(275ºC)
(250ºC)
I
33.4
16.2
43.4
10.2
8.1
22.7
m/z 39 (ketenimine)
m/z 50 (ketenimine)
m/z 78 (nitrile)
26.4
17.6
95.6
15.3
7.6
117.7
20.4
5.2
12.9
15.3
3.0
21.1
10.5
2.3
34.9
8.4
5.1
9.1
49.7
4.2
9.7
II
m/z 39 (ketenimine)
m/z 68 (ketenimine)
m/z 97 (nitrile)
68.9
28.9
7.7
60.1
24.2
14.3
53.2
23.5
25.0
37.1
30.0
9.1
30.4
21.3
13.1
26.2
20.1
25.2
31.4
25.9
8.3
23.4
19.4
12.5
17.2
14.3
18.3
III
m/z 39 (ketenimine)
m/z 66 (ketenimine)
m/z 122 (nitrile)
22.0
6.7
4.2
15.4
4.0
5.0
10.2
3.0
7.0
20.1
7.0
4.1
16.4
6.4
6.5
10.2
3.7
7.5
10.4
7.1
4.0
7.4
5.0
5.4
5.6
3.8
7.3
a
relative abundances are reported as (ion abundance×1000/total ion abundance) with 5% error. Independent duplicate determinations were carried out.
quantum chemical calculations are very good (see next section)
and constitutes strong support to the experimental findings.
in all the species considered here the lower energy form in
the ketenimine tautomer resulted from the proton migration
between the –cH2– and cºN groups.
Quantum chemical calculations
In a first step, geometry optimization of several nitrile and
ketenimine tautomers were calculated at the B3LyP method
with the modest 6-31G* basis sets. the most stable form for
Moreover, several conformations were calculated for each
nitrile and ketenimine forms. for the acid derivative, the possi-
bility of configurational isomerism arises, with the (E) form
being slightly preferred over the (Z) structure. A synperiplanar
all the studied systems correspond to the nitrile tautomer. conformation of the c=o and c=c double bonds, as well as for
for the studied species, tautomerization can be originated in the c=o and o–H bonds in the carboxylic group, is adopted
either the –cH3 or the –cH2 groups attached to the c=c bond. in the most stable conformer of both ketenimine and nitrile
Both alternatives were considered in the calculations and tautomers of the acid compound.

140
Determination of Heats of Tautomerization of Nitrile–Ketenimine
Figure 4. Effects of the sample introduction system tempera-
ture and the electron beam energy on ion abundance ratios for
2-sec-butylidenmalononitrile (I).
Figure 5. Effects of the sample introduction system tempera-
ture and the electron beam energy on ion abundance ratios for
2-cyano-3-methylpentan-2-oic acid (II).
for the ester species, a mutual syn conformation between
the c=o double bond and the o–c simple bond was found as
the most stable form, in accordance with previous results
reported for related methoxycarbonyl species.34
In a second step, the geometries of the minima were fully
optimized, including frequency calculations, with the B3LyP
(DH= Hketenimine − Hnitrile) show that the nitrile form is definitively
favored. the use of a split-valence triple-zeta basis set with the
inclusion of diffuse and polarization functions leads to slightly
lower differences in both energy and enthalpy of tautomerization.
regarding the experimental results obtained by mass spectrom-
etry, it is interesting to observe the consistency of the calculations
method using 6-31G* and the more extended 6-311++G** with the indicated fragmentation pathways. After applying the
basis sets.35
van’t Hoff equation [Equation (1)] to the slopes of figures 4–6, the
values for the experimental heats of tautomerization are in very
good agreement with the theoretical ones (table 3).
calculated relative energies (corrected for zero point
energy, DEo) and gas-phase tautomerization enthalpies

H. Saraví et al., Eur. J. Mass Spectrom. 17, 125–143 (2011)
141
have been considered in this and previous investigations9–23 but it
also provides experimental results for the determination of equi-
librium data for rapid reactions, provided that there is evidence
that these transformations do not occur or are insignificant
after ionization. the reported evidence found by mass spec-
trometry with regard to the occurrence of the nitrile–ketenimine
tautomerism has been supported through theoretical calcula-
tions. the value of mass spectrometry as a tool to predict the
occurrence of tautomerism has been demonstrated.
Acknowledgements
coDV and MfE to consejo Nacional de Investigaciones
científicas y tecnicas (coNIcEt) and the comisión de
Investigaciones científicas de la Provincia de Buenos Aires
(cIcBA), república Argentina. All the authors are indebted to
the facultad de ciencias Exactas, universidad Nacional de La
Plata for financial support.
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