π-systems as lithium/hydrogen bond acceptors: Some theoretical observations

TL;DR: In this article, the Hartree-Fock and correlated levels and density functional theory calculations have been performed with 6-31++G(d,p) and 6-311++G (d, p) basis sets on LiF and HF complexes of benzene, ethylene, and acetylene.

Abstract: Ab initio calculations at the Hartree–Fock and correlated levels and density functional theory calculations have been performed with 6-31++G(d,p) and 6-311++G(d,p) basis sets on LiF and HF complexes of benzene, ethylene, and acetylene Complex binding energies have been corrected for basis set superposition error, and zero point energy corrections have been done on Hartree–Fock binding energies Computed results indicate that the complexes exist in different conformations and among them those with π-lithium and π-hydrogen bonds are the most stable π-lithium bonds are stronger than π-hydrogen bonds The computed binding energies and geometry of HF complexes correlate well with the available experimental results LiF complexes with these π systems are found to be weaker than Li+ complexes but they are stronger than Li atom complexes Natural bond orbital analysis traces the origin of the weak interactions that stabilize the complex Li, as found in earlier cases, prefers the most symmetric site for interac

Summary

Introduction

• The computed binding energies and geometry of HF complexes correlate well with the available experimental results.
• Because of such a position of lithium the authors became interested in probing lithium bonds with different lithium bond acceptors, and as a first part they have investigated lithium bonded complexes with (n1p) donors andn donors, and presented their results elsewhere.

II. COMPUTATIONAL DETAILS

• All calculations have been carried out using the GAUSSIAN 94W program31 implemented on a Pentium computer.
• Basis set by selecting five possible geometries ~I–V! for the benzene complexes, three geometries~VI– VIII ! for the ethylene complexes, and two geometries ~IX–X ! for the acetylene complexes~.
• The secondary hydrogen bonding interaction between the protons of ethylene and the fluorine atom of HF/LiF are considered in structures VII~linear,Cs! and VIII ~bifurcated,C2v!.
• Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions.
• The complexation energies calculated at the Hartree–Fock level are corrected for both basis set superposition error~BSSE!.

III. RESULTS AND DISCUSSION

• HF and LiF form weak hydrogen/lithium bonded complexes with acetylene, ethylene, and benzene through F– H¯p and F–Lī p interactions and secondary hydrogen bonds.
• The results are discussed as follows: Potential energy surface, energetics, structure and bonding, and finally the analysis of interactions.
• Totally eleven structures have been proposed for these complexes and are presented in Figs. 1 and 2.
• The complex binding energies, BSSE, ZPEC, corrected binding energies, and the number of imaginary frequencies obtained for each structure of the complexes at the Hartree–Fock level are given in Table I and those at the DFT, MP2, and MP4 level in Table II.
• The results of NBO analysis for the monomers, and the complexes are summarized in Table IV.

A. Potential energy surface

• The p-bases employed here are highly symmetric and this limits the number of distinct interaction sites in the bases; consequently there are fewer interaction geometries for the complexes and this greatly reduces the scan time of the PES.
• The above bases can interact with LiF/HF by two ways;~i! through theirp electrons, that is the primary interaction and~ii !.
• Ethylene and acetylene have bond center and atom center as the primary interaction sites and ethylene has two secondary interaction sites and acetylene has one secondary interaction site.
• Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions.

1. LiF complexes

• In the p-lithium bonded structures~I, VI, and IX!.
• The p system acts as the donor and LiF as the acceptor and in the secondary hydrogen bonded forms~IV, VII, and X!.
• This will be of course treated as a separate case as similar forms are not observed in benzene and ethylene complexes.
• Such a trend is characteristic of hydrogen and lithium bonding situations.
• C2H4 and parallels the trend in the complex binding energies and this shows that the charge transfer plays a more dominant role in this hydrogen bonding situation.

2. HF complexes

• HF complexes of benzene, ethylene, and acetylene stabilize throughp-hydrogen bonds II, III, VI, and IX.
• Unlike LiF complexes, the secondary hydrogen bonded form is found to be stable only in the acetylene complex.
• The occupancies of the s* (H–F) andqCT values are much closer for II and III as the binding energies do.
• DE 2 shows that onep orbital interacts in II but a careful examination of the wave function reveals that bothp1 andp3 orbitals interact.
• A mixed donation fromns(F), np(F), and s~H–F! with greater participation ofnp(F) is observed;s* (C–H) orbital accepts the charge.

B. Energetics

• Complexation energies, BSSE, ZPEC, and corrected complexation energies computed at the HF level are compiled in Table I. Table II lists, the complexation energies, BSSE and BSSE corrected complexation energies computed at the DFT, MP2, and MP4 levels.
• ZPE corrections at the MP2, DFT levels could not be provided as calculations of frequencies at the MP2, DFT levels are computationally more demanding.
• For the same reason MP4 single point energies on MP2 geometries for benzene complexes are not reported.

C. Structure and bonding

• Complexation leads to changes that are significant only in selected structural parameters of the monomers and introduces new bonding parameters in the region of intermolecular contact.
• Selected MP2 parameters and Hartree–Fock frequencies of the monomers and complexes are listed in Table III.
• For the above analysis the data obtained with 6-3111G(d,p) are considered.

D. Analysis of interactions

• NBO analysis has been performed on Hartree– Fock wave functions of the stable forms with the 6-3111G(d,p) basis set.
• Computed orbital occupancies of the monomers, the quantum of charge transferred from donor to the acceptorqCT, occupancies of the frontier MOs, and DE2, the second order perturbation energy lowering due to the interaction of the donor and acceptor orbitals in the complexes are collected in Table IV.

IV. CONCLUSIONS

• The authors summarize the following conclusions from their theoretical investigations: ~i!.
• No significant changes are observed with the change of basis set.
• The stability order and quantum of charge transfer in these complexes indicate that though lithium and hydrogen bonds derive stability from electrostatic and charge transfer forces, lithium bonds have a more dominant contribution from the electrostatic interaction than hydrogen bonds do.
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• In the benzene complex, LiF prefers the most symmetric site for interaction where HF chooses a nonsymmetric location.

Figures

TABLE IV. Natural bond orbital analysis for the LiF and HF complexes calculated at the HF/6-3111G(d,p) level.

TABLE II. Interaction energiesDE, BSSE, counterpoise corrected interaction energiesDEcp (kcal/mol) for the complexes at the DFT, MP2, and MP4 levels.

TABLE III. Selected MP2 structural parameters and Hartree–Fock frequencies for the monomers and complexes optimized with the 6-3111G(d,p) basis set.

TABLE I. Interaction energiesDE, BSSE, counterpoise corrected interaction energiesDEcp, zero point vibration energy correction~ZPEC!, corrected binding energiesDEZPEC

FIG. 2. Optimized structures of C6H6¯HF ~II, III !, C2H2¯HF ~X!, and C2H2¯LiF ~XI ! complexes.

Full text

University of South Carolina University of South Carolina
Scholar Commons Scholar Commons
Faculty Publications Chemical Engineering, Department of
1998
π-systems as lithium/hydrogen bond acceptors: Some theoretical -systems as lithium/hydrogen bond acceptors: Some theoretical
observations observations
Salai Cheettu Ammal
University of South Carolina - Columbia
, ammal@cec.sc.edu
P. Venuvanalingam
Follow this and additional works at: https://scholarcommons.sc.edu/eche_facpub
Part of the Biological and Chemical Physics Commons
Publication Info Publication Info
Published in
The Journal of Chemical Physics
, Volume 109, 1998, pages 9820-9830.
Copyright 1998 American Institute of Physics. This article may be downloaded for personal use only. Any
other use requires prior permission of the author and the American Institute of Physics.
The following article appeared in
Ammal, S. S. C. & Venuvanalingam, P. (1998). π-systems as lithium/hydrogen bond acceptors: Some
theoretical observations.
The Journal of Chemical Physics
, 109, 9820. http://dx.doi.org/10.1063/
1.477651
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π-systems as lithium/hydrogen bond acceptors: Some theoretical observations
S. Salai Cheettu Ammal and P. Venuvanalingam
Citation: The Journal of Chemical Physics 109, 9820 (1998); doi: 10.1063/1.477651
View online: http://dx.doi.org/10.1063/1.477651
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p
-systems as lithium/hydrogen bond acceptors: Some theoretical
observations
S. Salai Cheettu Ammal
a)
and P. Venuvanalingam
b)
Department of Chemistry, Bharathidasan University, Tiruchirappalli-620 024, India
~Received 17 February 1998; accepted 2 September 1998!
Ab initio calculations at the Hartree–Fock and correlated levels and density functional theory
calculations have been performed with 6-3111G(d,p) and 6-31111G(d,p) basis sets on LiF
and HF complexes of benzene, ethylene, and acetylene. Complex binding energies have been
corrected for basis set superposition error, and zero point energy corrections have been done on
Hartree–Fock binding energies. Computed results indicate that the complexes exist in different
conformations and among them those with
p
-lithium and
p
-hydrogen bonds are the most stable.
p
-lithium bonds are stronger than
p
-hydrogen bonds. The computed binding energies and geometry
of HF complexes correlate well with the available experimental results. LiF complexes with these
p
systems are found to be weaker than Li
1
complexes but they are stronger than Li atom complexes.
Natural bond orbital analysis traces the origin of the weak interactions that stabilize the complex. Li,
as found in earlier cases, prefers the most symmetric site for interaction whereas proton prefers a
nonsymmetric site in benzene complexes. Surprisingly, such a change of interaction geometry in
LiF and HF complexes is found to change the donating
p
-orbitals in the benzene complexes.
© 1998 American Institute of Physics. @S0021-9606~98!30646-7#
I. INTRODUCTION
Intermolecular interactions play a significant role in sev-
eral biological
1
and chemical phenomena
2
and in view of
this, much attention has been focused in recent years to un-
derstand the physics and chemistry of such interactions
through experiment
3
and theory.
4
Mainly, the geometry and
strength of interaction have been the topic of interest and
very rarely.
5,6
the origin of such interactions have been
traced. Even hydrogen bonded systems that are abundant and
well studied are no exception to this. This is due to the rea-
son that, only recently, advanced theoretical procedures are
available and these enable one to analyze the interaction at a
more deeper level and derive novel insights from them.
Hydrogen bonded systems have been especially studied
due to their wide occurrence and relevance in biology and
chemistry. Lithium, congener to hydrogen, can form similar
bonds but comparatively lithium bonds are much less
investigated.
6–8
Although both, hydrogen and lithium bonds,
appear to be similar the type of interaction that stabilize the
former has been found to be different from that of the latter.
This has been observed
9
to have led to differential geometric
preferences and strength of interactions in hydrogen/lithium
bonded complexes. There are only very few reports on
lithium bonded complexes. Lithium has been grouped with
metal cations such as Na
1
,K
1
,Ca
21
, etc. in certain occa-
sions and in some other, it is considered as an isoelectronic
replacement of the proton. Thus lithium occupies an unique
position in that it forms weak bonds like hydrogen and at the
same time behave as metal ions like Na
1
,K
1
, etc. Because
of such a position of lithium we became interested in probing
lithium bonds with different lithium bond acceptors, and as a
first part we have investigated lithium bonded complexes
with (n1
p
) donors and n donors, and presented our results
elsewhere.
6
We have chosen here
p
systems—benzene, eth-
ylene, and acetylene as lithium bond acceptors. LiF is chosen
as the lithium donor. For comparison, we have performed
calculations on the complexes of HF with the same set of
donors; HF complexes of the above
p
bases have, already
been studied experimentally and at lower levels of theory.
X–H¯
p
interactions have been the subject of many
experimental
10–15
and theoretical investigations
16–24
and in
many occasions benzene, ethylene, and acetylene have been
employed as hydrogen bond acceptors. Flygare and
co-workers
10,11
have used microwave molecular beam tech-
niques to examine the structure of HF complexes with acety-
lene and ethylene and they have concluded that the complex
has T-shaped geometry with HF molecular axis pointing to-
wards the midpoint of the carbon–carbon bond. The above
results are fully consistent with ab initio predictions reported
by Pople and co-workers.
17
Though HF complexes of acety-
lene and ethylene have been investigated at various levels of
theory only few structural studies on the hydrogen bonded
complex with benzene as the H-acceptor have been carried
out due to complications of size.
The structure of the C
6
H
6
¯HF complex has been an
interesting subject for a long time but still the structure is a
matter of controversy. Baiocchi et al.
12
and Andrews et al.
14
have investigated the structure of the C
6
H
6
¯HF complex
using molecular beam electric resonance and infrared spec-
troscopy, respectively, and concluded that the complex has
a!
Present address: Department of Materials Chemistry, Graduate School of
Engineering, Tohoku University, Aoba-07 Sendai 980-8579, Japan.
b!
Author to whom correspondence should be addressed.
JOURNAL OF CHEMICAL PHYSICS VOLUME 109, NUMBER 22 8 DECEMBER 1998
98200021-9606/98/109(22)/9820/11/$15.00 © 1998 American Institute of Physics
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the C
6
v
equilibrium structure. An electrostatic potential map
of benzene shows that the
p
-electron cloud creates zones of
negative potential above and below the benzene plane and so
a favorable interaction occurs when the interacting dipole
lies along the C
6
ring axis with its positive terminus directed
toward the face of the ring. But the simple HOMO-LUMO
model for this interaction provides a conflicting structural
prediction since the symmetries of benzene HOMO and HF
LUMO do not favor C
6
v
geometry. Semiempirical CNDO
calculations
25
predict an asymmetric structure in which the
HF molecule mainly interacts with one of the CvC double
bonds. The reported ab initio calculations
18–20
with smaller
basis sets on the C
6
H
6
¯HF complex point to the symmetric
C
6
v
geometry. Single point MP2 calculations with larger ba-
sis set on this complex have been reported by Bredas and
Street
19
and Cheney and co-workers
18,20
and they have pre-
dicted the C
6
v
structure for the complex. These conflicting
predictions for the structure of the C
6
H
6
¯HF complex
stimulates further interest in this subject. Recently Rozas and
co-workers
24
have studied H¯
p
interactions using the Bad-
er’s AIM approach.
Lithium can form lithium bonds with unsaturated hydro-
carbons. The amount of experimental work done to date on
such
p
-lithium bonded complexes have been rather meager
and only theoretical reports are available.
6–8
Szczesniak and
Ratajczak
26
have reported ab initio calculations for the com-
plexes C
2
H
4
¯LiF and C
2
H
2
¯LiH. The complexes of Li,
Na, and K atoms with C
2
H
4
have been studied with coupled-
cluster and density functional theory ~DFT! methods by
Alikhani and co-workers.
27
The lithium ion affinity and
lithium atom affinity for benzene have been reported by Fujii
et al.
28
and Manceron and Andrews,
29
respectively. Ab initio
molecular orbital study at the HF and MP2 levels with the
6-31G
*
basis set on Li
1
complexes of first row bases, that
include ethylene and acetylene, have been done by Del Bene
and co-workers.
30
In continuation of our work on lithium bonded com-
plexes with (n1
p
) donors and n donors, we report here our
high level ab initio results on LiF/HF complexes of
p
do-
nors. The main objective of this paper is to ~1! observe how
the
p
systems respond to lithium donors ~2! look at the ge-
ometry around F–H¯
p
and F–Li¯
p
interactions ~3! trace
the origin of
p
-hydrogen and
p
-lithium bond interactions at
the orbital level.
II. COMPUTATIONAL DETAILS
All calculations have been carried out using the
GAUSSIAN 94W program
31
implemented on a Pentium com-
puter. The complex potential energy surface has been
scanned at the Hartree–Fock level with 6-3111G(d,p)
~Refs. 32–34! basis set by selecting five possible geometries
~I–V! for the benzene complexes, three geometries ~VI–
VIII! for the ethylene complexes, and two geometries
~IX–X! for the acetylene complexes ~Fig. 1!. In benzene
complexes, in the first three structures ~I–III! the
p
-
hydrogen/lithium bond interaction is the main force and in
the next two structures ~IV–V! hydrogen bonding interaction
involving benzene protons and the fluorine atom of HF/LiF
is the stabilizing force. Structure I has been optimized with
C
6
v
symmetry in which the HF/LiF molecule aligns with the
C
6
axis of benzene. Structures II and III have C
s
symmetry,
where in II the interaction of the HF/LiF molecule is mainly
with one of the CvC bonds and in III it is with one of the
carbon atoms of benzene. Structures IV and V consider, re-
spectively, the linear and bifurcated hydrogen bonding be-
tween the protons of benzene and the fluorine atom of HF/
LiF.
Among the three structures considered for the ethylene
complexes, structure VI has a T-shape geometry with C
2
v
symmetry in which the HF/LiF molecule interacts vertically
at the midpoint of the CvC bond. The secondary hydrogen
bonding interaction between the protons of ethylene and the
fluorine atom of HF/LiF are considered in structures VII ~lin-
ear, C
s
! and VIII ~bifurcated, C
2
v
!. There are only two pos-
sible orientations for the interactions of HF/LiF with acety-
lene; one with a T-shape geometry (C
2
v
) and another with
the linear hydrogen bonding interaction between the proton
of acetylene and fluorine atom of HF/LiF.
All the above mentioned structures of the complexes are
fully optimized within their symmetry constraints and the
harmonic frequencies for each structure have been calculated
at this level to characterize the stationary point. Geometry
optimizations have been carried out with larger triple-zeta
6-311G basis set
35
augmented by polarization
33
and diffuse
functions
34
@
6-31111G(d,p)
#
only for the stable struc-
FIG. 1. Proposed geometries of the LiF/HF complexes of benzene, ethylene,
and acetylene.
9821J. Chem. Phys., Vol. 109, No. 22, 8 December 1998 S. S. C. Ammal and P. Venuvanalingam
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tures. The stable structures obtained from Hartree–Fock
level have also been optimized at the MP2 and DFT level.
DFT calculations have been done with the exchange poten-
tial of Becke and correlation functional of Lee, Yang, and
Parr ~B3LYP!.
36
Inclusion of electron correlation at the
higher level have been carried out for the ethylene and acety-
lene complexes by calculating single point MP4 ~SDTQ! en-
ergies on the MP2 geometries. The complexation energies
calculated at the Hartree–Fock level are corrected for both
basis set superposition error ~BSSE! and zero point vibra-
tional energy ~ZPE! and those calculated at the DFT, MP2,
and MP4 levels are corrected only for the BSSE as frequency
calculations at these levels have not been done. BSSE has
been calculated using the Boys–Bernardi counterpoise
method
37
and also considering the relaxation of the monomer
geometries upon complexation.
38
Natural bond orbital
~NBO! analysis
39
on the stable forms of the complexes have
been carried out at the Hartree–Fock level.
III. RESULTS AND DISCUSSION
HF and LiF form weak hydrogen/lithium bonded com-
plexes with acetylene, ethylene, and benzene through F–
H¯
p
and F–Li¯
p
interactions and secondary hydrogen
bonds. The results are discussed as follows: Potential energy
surface, energetics, structure and bonding, and finally the
analysis of interactions. Totally eleven structures have been
proposed for these complexes and are presented in Figs. 1
and 2. The complex binding energies, BSSE, ZPEC, cor-
rected binding energies, and the number of imaginary fre-
quencies obtained for each structure of the complexes at the
Hartree–Fock level are given in Table I and those at the
DFT, MP2, and MP4 level in Table II. It should be noted that
MP2 calculations with triple zeta basis set and MP4 calcula-
tions with both double and triple zeta basis sets for benzene
complexes could not be done as they are computationally
more demanding. Selected MP2 structural parameters and
Hartree–Fock frequencies of the monomers, complexes are
listed in Table III. The results of NBO analysis for the mono-
mers, and the complexes are summarized in Table IV.
A. Potential energy surface
The
p
-bases employed here are highly symmetric and
this limits the number of distinct interaction sites in the
bases; consequently there are fewer interaction geometries
for the complexes and this greatly reduces the scan time of
the PES. The above bases can interact with LiF/HF by two
ways; ~i! through their
p
electrons, that is the primary inter-
action and ~ii! through their protons, that is the secondary
interaction. Benzene has three primary interaction sites, viz.,
face center, bond center, and atom center, and two secondary
interaction sites as shown in Fig. 1. Ethylene and acetylene
have bond center and atom center as the primary interaction
sites and ethylene has two secondary interaction sites and
acetylene has one secondary interaction site. Atom center
FIG. 2. Optimized structures of C
6
H
6
¯HF ~II, III!,C
2
H
2
¯HF ~X!, and
C
2
H
2
¯LiF ~XI! complexes.
TABLE I. Interaction energies DE, BSSE, counterpoise corrected interaction energies DE
cp
, zero point vibration energy correction ~ZPEC!, corrected binding
energies DE
ZPEC
cp
~kcal/mol!, and number of imaginary frequencies (n
i
) for the complexes calculated at the Hartree–Fock level.
Complex Structure
6-3111G(d,p) 6-31111G(d,p)
DE
BSSE DE
cp
ZPEC
DE
ZPEC
cp
n
i
DE BSSE DE
cp
ZPEC
DE
ZPEC
cp
n
i
C
6
H
6
¯LiF I 13.44 1.67 11.77 1.04 10.73 0 12.61 1.08 11.53 1.04 10.49 0
IV 2.91 0.34 2.91 0.38 2.19 0 2.98 0.30 2.68 0.22 2.46 0
V 2.60 0.37 2.60 0.23 2.00 2 ¯¯¯¯ ¯¯
C
2
H
4
¯LiF VI 9.70 1.11 8.59 1.15 7.44 0 8.70 0.45 8.25 1.03 7.22 0
VII 2.17 0.26 1.91 0.55 1.36 0 2.19 0.16 2.03 0.55 1.48 0
VIII 1.36 0.24 1.12 0.24 0.88 1 1.31 0.18 1.13 0.19 0.94 2
C
2
H
2
¯LiF IX 9.61 1.00 8.61 0.82 7.79 0 8.67 0.41 8.26 0.67 7.59 0
X 6.62 0.49 6.13 0.87 5.26 0 6.72 0.43 6.29 0.92 5.37 0
XI 9.58 0.55 9.03 0.85 8.18 0 9.29 0.33 8.96 0.88 8.08 0
C
6
H
6
¯HF I 2.90 0.24 2.66 0.61 2.05 2 ¯¯¯¯ ¯¯
II 2.98 0.26 2.72 0.85 1.87 0 3.11 0.47 2.64 0.85 1.79 0
III 2.98 0.26 2.72 0.84 1.88 1 3.11 0.46 2.65 0.81 1.84 0
IV 0.50 0.06 0.44 0.18 0.26 1 ¯¯¯¯ ¯¯
V 0.51 0.07 0.44 0.27 0.17 1 ¯¯¯¯ ¯¯
C
2
H
4
¯HF VI 3.18 0.19 2.99 1.57 1.42 0 3.06 0.27 2.79 1.45 1.34 0
VII 0.36 0.04 0.32 0.21 0.11 1 0.42 0.10 0.32 0.24 0.08 2
VIII 0.25 0.03 0.22 0.18 0.04 2 0.33 0.12 0.21 0.26 20.05 1
C
2
H
2
¯HF IX 2.99 0.13 2.86 1.29 1.57 0 2.95 0.25 2.70 1.26 1.44 0
X 1.48 0.17 1.31 0.44 0.87 0 1.60 0.27 1.33 0.38 0.95 0
9822 J. Chem. Phys., Vol. 109, No. 22, 8 December 1998 S. S. C. Ammal and P. Venuvanalingam
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Frequently asked questions

Q1. What are the contributions mentioned in the paper "Π-systems as lithium/hydrogen bond acceptors: some theoretical observations" ?

This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Ammal, S. S. C. & Venuvanalingam, P. ( 1998 ). 

Q2. What is the stable form of LiF?

In the most stable form LiF assumes a T-shaped geometry with ethylene and acetylene as HF does but with benzene, unlike HF, it prefers the symmetric C6v face centered geometry. 

Q3. What is the stability order of the lithium and hydrogen bonds?

The stability order and quantum of charge transfer in these complexes indicate that though lithium and hydrogen bonds derive stability from electrostatic and charge transfer forces, lithium bonds have a more dominant contribution from the electrostatic interaction than hydrogen bonds do. 

Q4. What is the p-bases used in the complexes?

The p-bases employed here are highly symmetric and this limits the number of distinct interaction sites in the bases; consequently there are fewer interaction geometries for the complexes and this greatly reduces the scan time of the PES. 

Q5. What is the order of stability among the p-lithium bonded structures?

The order of stability among the p-lithium bonded structures is found to be I.IX.VI. BSSE and ZPE corrected binding energies of these complexes show that the benzene complex is more stable while the other two vary very little in stability. 

Q6. What is the main reason for the stability of the p-lithium complex?

It has already been shown30 that the lithium bonding interaction derives its stability more from the ion-multipole interaction and the present context is evidence for such stabilization with the p-lithium acceptor. 

Q7. What are the two structures considered for the acetylene complex?

considered for the ethylene complex and the two structures ~IX and X! considered for the acetylene complex are found to be stationary points. 

Q8. What is the order of stability of the p-lithium bonded structures?

LiF complexes of benzene, ethylene, and acetylene are mainly of two types; one in which the complex is stabilized by the p-lithium interaction and in the other that is stabilized by the hydrogen bonding interaction involving the protons of p systems and the fluorine atom of LiF. 

Q9. What is the effect of the C–H bond length on the donor orbitals?

Slight contraction of the C–H bond length is also observed and this is a consequence of the C–H bond electron being pulled towards the donor orbitals, as found in H2CY¯Z–F (Y5O, S, Se; Z5H, Li! complexes. 

Q10. What are the BSSE and ZPE corrected binding energies?

BSSE and ZPE corrected binding energies show that p-lithium and p-hydrogen bond energies fall in the range 7.0–10.0 kcal/mol and 1.5–2.0 kcal/mol, respectively, and are weaker. 

Q11. What is the p-hydrogen bonded form of acetylene?

In addition to p-hydrogen bonded form acetylene has yet another form of C2H2¯HF complex; a form that is stabilized by the secondary hydrogen bond ~X!. 

Q12. What is the role of the NBO in determining the geometry and stability of the complexe?

NBO and direct analysis of wave functions have made it possible to unambiguously fix the origin of various interactions that stabilize the LiF and HF complexes of benzene, ethylene, and acetylene. 

Q13. What is the magnitude of the p-hydrogen bonding energy?

The magnitude of their computed binding energy shows that p-hydrogen bonds are weaker in general and weaker than p-lithium bonds inparticular. 

Q14. What is the difference between benzene and ethylene?

Unlike benzene and ethylene, acetylene is found to have yet another stable form XI on complexation with LiF; the form that is more stable than p-lithium ~IX! and hydrogen bonded ~X! forms, has a resting type or bent structure with the Li atom is placed at 2.4 Å above the CwC bond. 

Q15. What is the stable form of the C2H2 LiF complex?

The C2H2¯LiF complex has an additional form XI—the most stable among the three forms IX, X, and XI—stabilized by both the p-lithium interaction and the F¯H attraction. 

Q16. What is the distance between p and lithium bonds?

The calculated X¯Li distance in all three complexes is in the range of 2.0–2.4 Å and this shows that p-lithium bonds are stronger. 

Q17. What is the order of strength of the binding energies?

Uncorrected and BSSE corrected binding energies follow the same order of strength, ethylene.acetylene.benzene at the HF, DFT, and MP2 levels.