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Asian Journal of Chemistry;
Vol. 24, No. 10 (2012), 4243-4249
A DFT Study of Strecker Intermediates as Scavengers for Cyanides
C.A. FLORES-SANDOVAL1,*, F. GODINEZ-SALOMON1,2, J.M. HALLEN-LOPEZ2, H.I. BELTRÁN3,
R. HERNÁNDEZ-ALTAMIRANO1, D.A. NIETO-ALVAREZ4 and L.S. ZAMUDIO-RIVERA5,*
1Programa de Ingeniería Molecular, Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas 152, México D.F.
2Departamento de Ingeniería Metalúrgica, Escuela Superior de Ingeniería Química e Industrias Extractivas, IPN, Unidad Profesional Adolfo
López Mateos, México D.F.
3Departamento de Ciencias Naturales DCNI, UAM-Cuajimalpa, Pedro Antonio de los Santos 84, San Miguel Chapultepec, 11850, México
D.F.
4Programa de Recuperación de Hidrocarburos, Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas 152, México D.F.
5Laboratorio de Síntesis Química y Electroquímica, Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas 152, México D.F.
*Corresponding authors: Fax: +52 55 9175 6239; Tel: +52 55 9175 8269; E-mail: caflores@imp.mx; lzamudio@imp.mx
(Received: 4 February 2011;
Accepted: 26 April 2012)
AJC-11363
Bis-(aminomethyl)ethers (1-4) have been employed as molecular scavengers for cyanides. They are theoretically studied herein at DFT,
thus enabling a theoretical study in order to elucidate why the efficiency as scavenger depends on the substituent placed at the nitrogen
atom. The local softness (s+) value for C2 from electrostatic potential analysis (ESP) shows that the following decreasing order of
reactivity is in accordance with the experimental yield values: 4, 1, 2 and 3. 1290 runs considering four to ten parameters such as N3-C2
and C2-O1 bond lengths, HOMO and LUMO energies, GAP, dipolar moment, partition coefficient, atomic charge, local softness, global
hardness, electronegativity and 13C NMR chemical shift were taken into account in a structure-activity relationship study. In particular
five runs of linear combination of four (run 259), five (run 517), seven (runs 937 and 1063) and eight (run 1214) molecular indexes have
shown the best F values, being the run 517, which present the highest F value 4 to 10 parameters.
Key Words: Local softness, DFT, Molecular descriptor, Molecular reactivity, Scavenger.
INTRODUCTION
Metallic cyanides are among the most common corrosive
pollutants in crude oil1-3. In order to eliminate the cyanide
ions in the petroleum feedstock, some scavengers of cyanides
have been synthesized4. A strecker intermediates that act as
scavengers of cyanides were synthesized5; which have shown
different chemical reactivity, measured as reaction yield,
between 66 and 94 %, when cyanide is the nucelophile species
and thus this can be considered as their activity as cyanide
scavengers.
On another hand, the electron density (r) distribution is
a fundamental concept to understand the reactivity of molecular
entities and therefore, it can explain the phenomena of nucleo-
philic, electrophilic and radical attacks on a particular structure.
The frontier-electron density theory of chemical reactivity by
Fukui6 explained the importance of the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) in the chemical reactions. Therefore, the Fukui
function f(r) and local softness s(r) are of great interest in
chemical reactions, because these parameters could describe
the most reactive site in a given molecular species. The f(r)
function for a molecule can be defined as7:
f(r) = ( / (r) N = (r)/ N) (r) (1)
where, is the chemical potential, N is the number of elec-
trons, (r) is the potential acting on an electron due to all
nuclei and (r) is the electron density. The (r) measures how
sensitive a chemical potential is to an external perturbation at
a particular point. Thus,f(r) indirectly gives the reactivity of a
particular site and hence acts as a reactivity index. Considering
the fact that (r) is a discontinuous function of N, two diffe-
rent types of f(r) can be defined for any atom or molecule
corresponding to a nucleophilic and electrophilic attack.
However, the evaluation of these f(r) values is quite intricate.
A simple procedure to calculate fk (condensed Fukui function)
was proposed by Yang and Mortie, an alternative for f(r), based
on the Mulliken population analysis (MPA) and the finite-
difference approximation of eqn. (1). In a finite-difference
approximation, for a system of N electrons, the f(r) values are
given as:
f+(r) = qk(N0 + 1) - qk(N0) for a nucleophilic attack (2)
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(%)
4244 Flores-Sandoval et al.
Asian J. Chem.
f(r) = qk(N0) - qk(N-1) for an electrophilic attack
(3)
sodium cyanide, in water and solvent mixtures, that have a
where, qk(N0), qk(N0-1) and qk(N0 + 1) represent the electronic
composition similar to -crude oil; in where the efficiency (Eff)
population in the k atom for the N0, N0-1 and N0+1 electron
for scavenging were of 90.7, 65.8, 58.8 and 93.5 % for 1, 2, 3
system, respectively.
and 4, respectively. The last one has a pseudo-ephedrine
The local softness parameter is defined as:
fragment (the most branched N substitution in the whole series)
s(r) = f(r)S
(4)
and it shows the highest efficiency as scavenger. Thus, the
where, S is the global softness. Furthermore, the three appro-
experimental yield values depend on the structure and thus
ximate atomic f(r) indexes, when they are multiplied by S,
the reactivity of the employed
-amino alcohols, not only to
provide two different local softnesses for any particular atom
form the Strecker intermediates but also to scavenge cyanide
(k), which can be written as:
ions in order to obtain N substituted glycine derivatives as the
sk+ = [qk(N0 + 1) - qk(N0)] S
(5)
non-pollutant species. In this contribution, a theoretical study
sk_ = [qk(N0) - qk(N0-1)] S
(6)
of these compounds was carried out in order to observe the
Finally, the
f(r) descriptor defined as the difference of the
effect of having different molecular fragments linked to C2
nucleophilic Fukui function f+(r) and the electrophilic Fukui
atom (Fig. 1), over some molecular parameters. The under-
function f_(r)8. The sign of the
f(r) describes the electrophile
lined importance over this atom is because of it is flanked by
( f(r) > 0) and nucleophile ( f(r) < 0) zones within a molecule.
two, more electronegative, atoms (N3-C2-O1) and since it
On the other hand, it is well known that quantum chemistry
is the electron deficient site, due to double neighboring
enables tools to obtain chemical descriptors (molecular orbital
inductive effect and hence it is also the reactive site. In this
energies, gap, atomic charge, etc.) these can be employed to
line C2 atom acts as electrophilic site and therefore, it suffers
carry on quantitative structure-activity relationships (QSAR).
the nucleophillic attack of the cyanide ion; due to the latter
This analysis has been widely employed in the biological area,
facts the quantum chemical parameters are hereafter referred
however, some works have been published in the field of
to this atom.
corrosion science9-12, or other scientific and industrial fields
where a particular activity is acknowledged and is intended to
be optimized in such a way.
In this work, we present a study performed at DFT for
four already known and tested molecular scavengers. This
survey takes into account the analysis of reactivity parameter
(local softness) in order to relate this with the scavenging
activity. In this work, we have done a structure-activity study13
in order to account the factors that govern such activity.
EXPERIMENTAL
DFT calculations implemented in the Gaussian 98 program
package14 were done using to carry out the optimization of the
structures by using a B3LYP functional15. The B3LYP hybrid
Compound R1 R2 R3 R4 Yield
1 7CH2-Ph H H H 91
2 7CH2CH2CH2CH3 H H H 66
3 7C(CH3)3 H H H 59
4 7CH3 CH3 H C6H5 92
Fig. 1. Structure of (N,N-dialkyl-amine)methyl ethers used in the study
functional defines the exchange functional as a linear combi-
Table-1 listed the geometric parameters from optimized
nation of Hartree-Fock, local and gradient-corrected exchange
structures. Fig. 2 showed the optimized structures. As can be
terms15-19. The 6-31G+(d) basis set was employed in this work,
seen from Fig. 2, all the structures present a symmetry-plane
where diffuse functions were employed because they are known
(
) that cross through the O1 atom. The presence of this
to supply a better energy value for anionic species. The zero-
symmetry plane in the optimized structures is in accordance
point energy correction (ZPE) was calculated at same level of
with the signal pattern observed in the experimental 1H and
theory in all four molecular cases under study.
13C NMR spectra5. In compound 4 there exists an interesting
The 13C NMR shielding tensors were calculated using
proximity of the aromatic rings, in which the HA-HB bond
the methodology of Gauge Independent Atomic Orbitals
distance was of 2.326 Å, whereas in 1 the bond distance
(GIAO)20-23.
between both aromatic rings was of 6.82 Å. This bond distance
The isotropic shielding values defined as:
is shorter in 4 due to ethylene moiety, which is bonded to the
iso = 1/3 ( 11 +
22 +
33)
(7)
aromatic rings and lends more flexibility for this fragment,
where (
ii being the principal tensor components) were used
whereas in 1, this flexibility is more restricted due to the
to calculate the isotropic chemical shifts ( ) relative to TMS
presence of a benzyl moiety (R1 moiety). Thus, when other
( X =
TMS -
X)24-26. Finally, the parameters for the structure-
CH2 unit was added to the benzyl fragment (this implies
activity study were obtained at B3LYP/6-31G+(d) level,
another degree of freedom in the chemical structure), the
except the log P parameter, which was obtained by using Hyper
distance between both aromatic rings decreased approximately
Chem software.
to 1.7 Å. In this case, both rings are almost in the same plane.
RESULTS AND DISCUSSION
Then, the addition of an ethylene fragment causes an inherent
proximity of aromatic rings. Furthermore, from Table-1, the
The synthesis of Strecker intermediates as bis-(amino-
presence of n-butyl or t-butyl fragment provokes a decrease
methyl)ethers (1-4) 1-4, have already been reported (Fig. 1)5.
in the energy value, which could be attributed mainly to non-
Such compounds could act as non-pollutant scavengers for
bonded interactions like steric hindrance.