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The Journal of Argentine Chemical Society
versión On-line ISSN 1852-1428
J. Argent. Chem. Soc. vol.96 no.1-2 Ciudad Autónoma de Buenos Aires ene./dic. 2008
REGULAR PAPERS
The electronic structure of the C2H3Cl···HX (X = Cl, CN, NC and CCH) vinylcloride complexes evaluated via topological parameters and DFT calculations
Boaz G. Oliveira
Departamento de Ciências
Farmacêuticas, Universidade Federal de Pernambuco
50740-521, Recife – PE, Brazil
E-mail: boazgaldino@gmail.com
Received
August 11, 2008.
In final form November 12, 2008.
Abstract
The optimized geometries of the C2H3Cl···HX
vinylchloride complexes with X = Cl, CN, NC and CCH were obtained at B3LYP/6-311++G(d,p)
level of theory. In a first analysis, the (Cl···HX) hydrogen
bonds were examined by taking into account their distances, intermolecular interaction
energies and CHELPG charge transfer, as well as charge densities obtained through
the calculations of theory of Atoms In Molecules (AIM). Moreover, a (X···H)
secondary interaction between the hydrogen atoms of vinylchloride and fluorine
of HF, nitrogen of both HCN and HNC molecules, and p bond
of HCCH was also a target of investigation. This secondary interaction was studied
not only in terms of topological criteria or by results of structural and electronic
parameters, but it was also analyzed the stretch modes in the infrared spectrum.
At last, as far as the vibrational red-shifts of the HX molecules were characterized,
the stretch frequencies and absorption intensities of the (X···H)
secondary interactions were also identified, but only for the C2H3Cl···HCCH
complex.
Resumen
Las geometrías del C2H3Cl···HX vinylchloride complejos
con X = Cl, CN, NC y CCH fueron obtenidas en el nivel de teoría B3LYP/6-311++G(d,p).
En un primer análisis, el ligaciones de hidrógeno (Cl···HX)
fueron examinadas teniendo en cuenta sus distancias, energías de interacción
intermoleculares y transferencia de carga CHELPG, así como las densidad
de electróns obtenidas por los cálculos de la teoría de
Atoms in Moleculres (AIM). Además, una interacción secundaria
(X···H) entre los átomos de hidrógeno de
vinylchloride y el flúor de HF, nitrógeno de HCN como de las moléculas
de HNC, y la ligacion p de HCCH fueron también
un objetivo de investigación. Esta interacción secundaria fue
estudiada no sólo en términos de criterios topológicos
o por resultados de parámetros estructurales y electrónicos, pero
también fue analizado los modos de extensión en el espectro infrarrojo.
Por fin, por lo que los cambios rojos vibrational de las moléculas HX
fueron caracterizados, las frecuencias y intensidades de absorción del
las interacciones secundarias (X•••H) también fueron
identificadas, pero sólo para el complejo C2H3Cl
••• HCCH.
Introduction
Nowadays, it has been manifested a great interest by studies of non-covalent
intermolecular phenomena, in which the hydrogen bonding is one of the most important
types because there is a large number of biological, physical and chemical systems
formed through the interaction between a proton donor and a rich charge density
site. One important charge density center is the halide species, which in some
situations interacts with hydrocarbons and produces the halide hydrocarbons.
As example, vinylhalide or more specifically vinylchloride (C2H3Cl),
which is a chloride hydrocarbon containing a carbon-carbon double bond formed
by attack of hydrochloric acid to acetylene [1-2], properly
the formation of C2H3Cl is ruled by the interaction of
the hydrochloride acid aligned with p bond of acetylene,
what leads at formation of an intermolecular system characterized by a typical
(p···H) interaction [3].
After this, other hydrochloride acid molecule interacts with the n lone electron
pair (Cl) of vinylchloride and thereby a (n···H) hydrogen
bond is formed [4].
Some years ago, geometries and related parameters of p
intermolecular systems formed by acetylene and proton donors have been detailed
studied [3]. Recently, coexistence of theoretical and experimental
observations about the electronic structures of the C2H3Cl···HCl
and C2H3Cl···HF complexes has been
reported by Herrebout and van der Veken [5-7]. Experimentally,
by using the pulsed-nozzle Fourier Transform Microwave Spectroscopy (FTMS) [8],
Legon and co-workers have documented several studies about the equilibrium geometry
of hydrogen complexes constituted by vinylhalide and monoprotic acids [9-11].
From these works, it has been established that the interaction between vinylhalide
(Y = Cl or F) and acid species (HX) are characterized by a distortion on its
(Y···HX) hydrogen bonds, as already observed in other hydrogen-bonded
species [12]. Specifically, the (Y···HX)
hydrogen bond is non-linear because is supposed the formation of a secondary
interaction between the halogen and hydrogen atom of vinylhalide. For C2H3Cl···HCl
and C2H3F···HF complexes, in fact
this secondary interaction is based on experimental evidences, which says that
Cl and F interacts with the H atom (=CH2 group) of the C2H3Cl
and C2H3F molecules, respectively [11].
By taking into account the C2H3Cl···HX
with X = Cl, CN, NC and CCH set of complexes, however, we alert the importance
for studying the nature of the secondary interaction in these systems. In practice,
we expect to obtain theoretical informations that corroborates with the FTMS
experimental results, as well as lead us to predict new insights about this
interaction phenomenon.
Computational strategy
However, by projecting a theoretical study of the C2H3Cl···HX
vinylchloride complexes and their hydrogen bonds and secondary interactions,
we should mention that a quantumchemical description of charge density and molecular
topology seems to be essential. The objective of determination and evaluation
of molecular surface parameters from charge density can be naturally associated
to theory of Atoms in Molecules (AIM) [13] of Bader. In this
work, the contribution of AIM calculations will be performed by localization
of Bond Critical Points (BCP) between chloride of vinylchloride and hydrogen
atom of acid species, as well as if possible between Cl, CN, NC and CCH groups
and hydrogen atoms of the vinylchloride, what properly characterizes the (Cl···X)
hydrogen bonds and (X···H) secondary interactions, respectively.
In this insight, we would like to emphasize that the choice of computational
method must be evaluated critically. As well-known, the addition studies of
hydrogen halides to alkenes [14] have been effectuated successfully
by using the Density Functional Theory (DFT) [15]. In terms
of DFT calculations, the B3LYP [16] hybrid was chosen because
a lot of hydrogen-bonded systems have been successfully studied through this
popular functional [17-23]. For ab initio basis sets, during
all these years the choice for split-valence and/or correlation consistent types
still yields intense discussion among the theoreticians. Thereby, we admit some
traditional concepts, i.e., the description of "heavy" (C, N and
Cl) and "soft" (H) atoms will be developed through the application
of "d" and "p" polarization functions within the 6-31G
Pople's split-valence wave function. Fundamentally, the inclusion of diffuse
functions (++) is also important, mainly for BSSE correction.
Theoretical
procedure
The optimized geometries of the C2H3Cl···HX
with X = Cl, CN, NC and CCH vinylchloride complexes were processed by using
the GAUSSIAN 98W program [24] through the execution of the
B3LYP/6-311++G(d,p) calculations. Through the GAUSSIAN 98W, it was calculated
the CHELPG atomic charges [16]. The DQCHELPG
quantities were determined by a balance of atomic charges on proton donor molecules.
The topological analysis was performed by using the AIM 1.0 2000 program [25].
The results of DE interaction energies were determined
through the application of the supermolecule approach, DE
= E(AB) – [E(A) + E(B)] [26]. Furthermore, the DE
values were corrected by the BSSE [27] scheme of Boys and
Bernardi.
Results
and discussion
Optimized geometrical parameters
From the B3LYP/6-311++G(d,p) calculations, the optimized geometries of the C2H3Cl···HCl
(I), CC2H3Cl···HCN (II),
C2H3Cl···HNC (III) and C2H3Cl···HCCH
(IV) vinylchloride complexes are illustrated in Figure
1, whereas the results of the structural parameters are listed in Table
1. In this picture, the van der Waals surfaces are represented in background
[28]. From a long time ago, more specifically since the first
empirical studies of hydrogen complexes, the van der Waals fields have been
taken as useful parameter to analyze intermolecular interactions and thereby
some interesting conclusions were proposed [29-33]. For the
vinylchloride complexes studied here, it was admitted that van der van Waals
surface can contribute at least qualitatively with the study of the hydrogen
bonds (Cl···X) and secondary interactions (X···H).
Thus, the van der Waals surfaces depicted in Figure 1 shows that (X···H)
secondary interactions are not formed, although almost all (Cl···H)
hydrogen bonds were identified, excepting in the C2H3Cl···HCCH
vinylchloride complex because the van der Waals surfaces of HCCH and vinylchloride
does not overlaps. By assuming that van der Waals surface is an parameter of
current interest in studies of molecular modeling and intermolecular interactions
[34], in practice the van der Waals radii based on this surface
are the only criteria to certify the formation of an intermolecular contact,
although if we consider the R(Cl···HX) distances of 2.5471
Å, 2.7704 Å, 2.464 Å and 3.035 Å for (I), (II),
(III) and (IV) vinylchloride complexes, as well as their R(X···H)
secondary interactions values of 3.3599 Å, 3.8470 Å, 3.249 Å
and 3.1589 Å, excepting the C2H3Cl···HCCH
whose R(Cl···HX) and R(X···H) results
are longer than sum of van der Waals radii [H (1.40), C (1.70), O (1.50), and
Cl (1.76)], all remaining complexes have typical intermolecular distance values.
Fig. 1. Illustration
of the van der Waals surfaces and optimized geometries of the
C2H3Cl···HCl (I), C2H3Cl···HCN
(II), C2H3Cl···HNC (III) and C2H3Cl···HCCH
(IV)
vinylchloride complexes using B3LYP/6-311++G(d,p) calculations.
Table 1. Main structural
parameters of the C2H3Cl···HX vinylchloride
complexes using B3LYP/6-311++G(d,p) calculations.
* Values of r and R are
given in ångströms, Å;
* Values of q angles are given in degrees.
A natural consequence of secondary interactions for vinylchloride complexes is the non-linearity deviation (q) on the (Cl···X) hydrogen bonds (35-36). From B3LYP/6-311++G(d,p) calculations, it was found a large q value of 28.7 º for (IV), whereas smaller for (I), (II) and (III) complexes. Based on previous papers (37), the linearity deviation is observed when the distances of the secondary interactions are compatible to van der Waals radii. Although all R(X···H) values are longer than van der Waals radii, this distance criterion is not adequate for (I), (II), (III) and (IV) complexes. Regarding to molecular deformations, the slight DrHX value of 0.0014 Å for (IV) indicates how weakly bound is this intermolecular system. Moreover, for (I) and (III), we can observe by DrHX results of 0.006 Å and 0.0063 Å that the hydrochloride acid and hydroisocyanic acid properly provokes substantial deformations on the structure of the vinylchloride complexes.
Interaction energies
and charge transfers
The values of uncorrected intermolecular energies (DE),
corrected intermolecular energies (DEC),
and BSSE correction amounts for the (I), (II), (III) and
(IV) vinylchloride complexes are presented in Table 2. All DEC
results obtained here corresponds to low energies, which can be associated to
van der Waals interactions whose values are in range of 3.0-9.8 kJ mol-1 [38].
In this sense, we can emphasize that these DEC
results were obtained by small BSSE contributions, as in accord with data documented
for p hydrogen-bonded complexes [39].
As debated here, the main structural changes (DrHX)
which occurs due to formation of hydrogen complexes is related to proton donor
molecules. Such geometric parameter suggests systematic tendencies about the
interaction strength by means of a direct relationship between DrHX
results and DEC intermolecular energies,
as shown by Figure 2. From this picture, a linear coefficient
R2 of 0.96 was computed, what in fact, is a demonstration that stronger
vinylchloride bound complexes are formed by intense changes on their molecular
structures (40). In fact, by the interaction between HOMO
orbital of proton acceptor and LUMO orbital of proton donor (41),
electronically we can interpret the charge transfer between these frontier orbitals
by means of the intermolecular CHELPG quantities (DQCHELPG).
From DQCHELPG values of -0.026e.u. and
-0.036 e.u. gathered in Table 2, we can verify the (I)
and (II) complexes as the strongest bound systems, what corroborates
with DEC Sanalysis already debated here.
For (III) and mainly the (IV) complex, the DQCHELPG
amounts of -0.014 e.u. and -0.012 e.u. indicates weak interaction strengths.
Fig. 2. Relationship
between the values of the corrected intermolecular energies (DEC)
and increments of the proton donor bonds (DrHX) of
the
C2H3Cl···HCl (I), C2H3Cl···HCN
(II), C2H3Cl···HNC (III)
and C2H3Cl···HCCH (IV) vinylchloride
complexes using B3LYP/6-311++G(d,p) calculations.
Table
2. Values of uncorrected (DE) and corrected intermolecular energies (DEC),
BSSE amounts and CHELP charge transfer
(DQCHELPG) of the C2H3Cl···HX vinylchloride complexes using
B3LYP/6-311++G(d,p) calculations.
* Values
of DE, BSSE and DEC are given in kJ mol-1.
Infrared spectra
analysis
In terms of infrared spectrum parameters, the main vibrational modes of the
(I), (II), (III) and (IV) vinylchloride complexes
are presented in Table 3. As widely known, one of the most
important vibrational phenomena upon the formation of hydrogen complexes is
the red-shift effects on the proton donor [42], whose n(HX),m
frequency signal of isolated monomer is re-localized to a lower n(HX),c
region accompanied by a elevation on the absorption intensity, I(HX,c)/I(HX,m)
(35). For the (I), (II), (III) and (IV) vinylchloride
complexes, their Dn(HX) results were analyzed by
DQCHELPG amount data, whose relationship
between them is in very good agreement, as can be seen in Figure
3. By analyzing this graph, a R2 linear correlation coefficient
of 0.95 says that the red-shifts can be securely interpreted by quantification
of the intermolecular charge transfer.
Table 3. Main vibrational
modes of the C2H3Cl···HX vinylchloride
complexes using B3LYP/6-311++G(d,p) calculations.
* Values of n and I are given in cm-1 and km mol-1, respectively;
* Dn(HX) = n(HX),c
- n(HX),m.
Fig.
3. Relationship between red-shift effects of HX proton donating and CHELPG
charge transfer on
C2H3Cl···HCl (I), C2H3Cl···HCN
(II), C2H3Cl···HNC (III)
and C2H3Cl···HCCH (IV) vinylchloride
complexes.
Well, the excellent relationship for charge transfer is because the CHELPG algorithm has been implemented to calculate electronic populations near of outside of the van der Waals surface [43-45], which is illustrate on Figure 1. However, it was also verified the existence of weak vibrational modes, which in essence are named as hydrogen bond stretch frequencies. So, we can verify by the n(Cl···HX) and I(Cl···HX) values that all vinylchloride complexes are weak bound, mainly for (IV) whose result of absorption intensity of 0.65 km mol-1 is the lowest. However, it was found an interesting information for (IV) concerning to secondary interaction stretch frequency, whose results for n(X···H) and I(X···H) are 21.1 cm-1 and 1.3 km mol-1, respectively. In fact, the discovering of this secondary interactions is quite impressive, although we should notice that such observation was not verified on the (I), (II) and (III) complexes. In terms of the optimized geometric structure, the secondary interaction on (IV) was expected due the larger non-linearity deviation (q) computed.
Contribution
of the AIM topology
Regarding to electronic parameters, we hope that an investigation of the molecular
charge density can help us to understand the possibility to form the (Cl···H)
hydrogen bonds and (X···H) secondary interactions. In this
insight, the Table 4 presents the results of AIM calculations
for the (I), (II), (III) and (IV) vinylchloride
complexes, where the values of electronic densities (r)
and Laplacian (Ñ2r)
are presented. According to inherent approaches of the AIM theory, Ñ2r
Laplacian is a topological parameter defined through the sum of the eigenvalues
from diagonalization of Hessian matrix of r. Through
the identification of Bond Critical Points (BCP) between two neighbouring atoms
(46), Laplacian of r can be used
to characterize the formation of hydrogen-bonded complexes when concentrations
(Ñ2r> 0)
and depletions (Ñ2r
< 0) of the charge density are quantified. If take a look in our results,
we can firstly perceive the high charge density concentration on HX bonds, whose
values are defined in an average of 0.190 e/a03. Moreover,
the negative Laplacian values also indicates an electronic accumulation on proton
donor bonds, what in terms of AIM theory, such elevated lumps of r
is named as shared interactions. In other words, the charge concentration
is localized within the atomic pathway, more specifically on BCP.
Table 4. Values of
electronic densities (n) and Laplacians (Ñ2r)
of the C2H3Cl···HX vinylchloride
complexes using AIM calculations.
For the (Cl···HX) hydrogen bonds, however, the AIM result shows a slight concentration of charge density, consequence of an electronic separation on the atomic centers (Cl and H) revealed by negative values to Ñ2r (47), as can be seen in relief maps of electronic density of the (I), (II), (III) and (IV) vinylchloride complexes pictured in Figure 4. Indeed, we can see a protuberance of charge density on Cl atom of vinylchloride and X (Cl, CN, NC and CCH) groups of the proton donors. Nevertheless, as previously debated about the possibility to form the secondary interaction between the hydrogen atoms of vinylchloride species and CN, NC and CCH groups of proton donors, as demonstrated in Table 4 the r(X···H) and Ñ2r (X···H) values of 0.003 e/a03 and 0.008 e/a05 obtained from AIM integrations identified such secondary interactions, but only on the C2H3Cl···HCCH complex (IV), as demonstrated in Figure 5, where is illustrated their whole set of BCP.
Fig. 4. Relief maps
of the electronic densities on the C2H3Cl···HCl
(I), C2H3Cl···HCN (II),
C2H3Cl···HNC (III) and C2H3Cl···HCCH
(IV) vinylchloride complexes.
Fig. 5. BCP (in small black spheres) and molecular pathways on the C2H3Cl···HCl (I), C2H3Cl···HCN (II), C2H3Cl···HNC (III) and C2H3Cl···HCCH (IV) vinylchloride complexes.
However, we make evidence that the secondary interaction is not formed between the hydrogen atom of vinylchloride and r bond of acetylene, by contrary, it was found that the hydrogen of vinylchloride interacts directly with carbon of acetylene instead of unsaturated bond [11]. As discussed in the structural parameters that even though the CCH···H distance is longer than sum of van der Waals radii, but the AIM protocol was efficacious to identify a BCP between the C and H atoms, showing then the existence of the secondary interaction, which has been presumed by FTMS analysis [10]. By the way, it is not surprising that BCP are localized between near atoms, as in case of halogen interactions recently fond and reported by Grabowski [49]. So, a direct consequence about the identification of the secondary interaction in (IV) is that, undoubtedly its interaction energy of 2.28 kJ mol-1 cannot be associated to Cl···H primary hydrogen bond, but it must be also related to CCH···H. Thereby, there is a limitation of the supermolecule approach for determination of the interaction energies, which cannot be performed by subtracting the energy of the complex (C2H3Cl···HCl, C2H3Cl···HCN, C2H3Cl···HNC or C2H3Cl···HCCH) minus of its isolated monomers (C2H3Cl and HCl, HCN, HNC or HCCH). This problem was recently debated in hydrogen-bonded complexes (alcohol···water and heteorcyclic···hydrofluoric-acid) [49-50] wherein multiple hydrogen bonds were characterized on these structures.
Concluding
remarks
A theoretical study about the molecular structure of the C2H3Cl···HX
vinylchloride complexes with X = Cl, CN, NC and CCH was proposed through the
B3LYP/6-311++G(d,p) calculations and topological integrations of the atoms in
molecules theory. Through the structural and electronic parameters, the vinylchloride
complexes were considered weakly bound systems because it was computed large
R(Cl···H) hydrogen bond distances and low DEC interaction
energies.
Based on experimental assays documented in literature, it has been observed
a (X···H) secondary interaction formed between the halogen
atom (H) of vinylchloride specie and X group of the HCl, HCN, HNC and HCCH monoprotic
acids. From B3LYP/6-311++G(d,p) calculations, this secondary interaction unlikely
cannot be formed because the R(X···H) distance is longer
than sum of van der Waals radii. Still about this, it was showed by van der
Waals surfaces that no secondary interactions might to exist. Indeed, by taking
into account the van der Waals surface of the C2H3Cl···HX
set of complexes, as far as the (X···H) secondary interactions
is not observed, but for C2H3Cl···HCCH system, the structure
of its optimized geometry suggests that the formation of the (Cl···H)
primary hydrogen bonds also is not possible to be done.
By analyzing the infrared spectrum, however, we have observed that all n(Cl···H)
stretch frequencies and I(Cl···H) absorption intensities
were successfully identified, although the n(X···H)
signal of the secondary interaction on C2H3Cl···HCCH
complex was characterized and interpreted. In corroborating to that, AIM calculations
also localized BCP between CCH group and vinyl hydrogen, although this contact
is formed by a direct alignment of vinyl hydrogen to the carbon of acetylene
instead of r bond.
Supplementary
information
Cartesian coordinates of optimized geometries and the values of the molecular
energies (E) of the C2H3Cl···HCl
(I), C2H3Cl···HCN (II),
C2H3Cl···HNC (III) and C2H3Cl···HCCH
(IV) vinylchloride complexes using B3LYP/6-311++G(d,p) calculations.
Acknowledgements
The author would like to gratefully to the CAPES and CNPq Brazilian funding
agencies, as well as to CENAPAD-SP (Centro Nacional de Processamento de Alto
Desempenho-São Paulo) by computational facilities to complete this work.
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