Shiraz University
13th Iranian Physical Chemistry Seminar
Shiraz University of Technology
Intramolecular hydrogen bonding and vibrational spectra of 2,2,6,6-tetramethyl-3,5-heptanedione
M. Vakili
a
a,*
, H. Miremad b, S. Salemi a, A-R. Nekoei
c
Department of Chemistry, Sabzevar Tarbiat Moallem University, Iran
E-mail:* mvakilli@yahoo.com
8H
b
School of Chemistry, Damghan University of Basic Sciences, Damghan, 36715-364, Iran
c
Department of Chemistry, Shiraz University of Technology, Shiraz, 71555-313, Iran
Keywords: 2,2,6,6-tetramethyl-3,5-heptanedione; -dicarbonyl; Conformational analysis, Vibrational assignment; Intramolecular
hydrogen bond; Density Functional Theory.
1. Introduction
It is well known that the cis-enol form of a -dicarbonyl is characterized by an intramolecular hydrogen bond. Any factor which affects
89H
on the electronic properties of the chelated ring can change Intramolecular hydrogen bond strength (IHB) and the tautomerization
equilibrium [1-4]. To the best of our knowledge structure and vibrational spectra of 2,2,6,6-tetramethyl-3,5-heptanedione (TMHD),
theoretically or experimentally, were not described earlier. The aim of the present paper is to predict intramolecular hydrogen bond
strength and vibrational spectra of TMHD by means of density functional theory (DFT) levels.
2. Experimental and method of analysis
TMHD compound purchased from Aldrich chemical company. D2-TMHD was prepared by mixing the TMHD with D2O. The FT-IR,
FT-Raman and Far-IR spectra were obtained on a Bomem MB-154 and Thermo Nicolet NEXUS 870 FT-IR spectrometers. All
calculations were performed using Gaussian 03W package program, B3LYP/6-311++G** and MP2/6-31G** levels of theory for
optimizing all possible cis-enol conformations and B3LYP/6-311G** level for calculating vibrational frequencies of the stable cis-enol
forms.
3. Results and Discussion
Among 32 possible enol conformers of TMHD, only four conformers have the chelated structure. These four conformers and the
atom numbering are shown in Fig.1. The B3LYP and MP2 calculations show that only two of these conformers are stable. The
III and IV conformations are not stable, and under optimization turn to I and II. The energy difference between these stable
chelated enol forms is 0.15-0.53 kcal/mol, calculated at B3LYP/6-311++G** and MP2/6-31G**. So, from the theoretical point
of view, the coexisting of both cis-enol conformers (I and II), at least in the gas phase, is suggested. Molecular geometry,
intramolecular hydrogen bond energy, EHB (the energy difference between the chelated cis-enol and non-chelated trans-enol
rotamers), and experimental vibrational frequencies related to IHB of TMHD are given in Tables 1 and 2. Comparing coordinates
for TMHD and AA shows that upon substitution of t-But groups in the positions and because of the more steric effects of
these groups, the bond angles O1C2C8 and O5C4C21 in TMHD are smaller than those of acetylacetone (AA). This fact
decreases the OO distance and increases the IHB of TMHD in comparison with AA (Table 1).
Fig.1. Numbering and Possible chelated cis-enol tautomers of TMHD.
The averaged values of ROO and OHO for the chelated rings of Benzoylacetone (BA), AA and TMHD, EHB and some
spectroscopic properties, related to their IHB, are listed in Table 2. After comparing these data, the following trend in hydrogen
bond strength is concluded:
TMHD>BA>AA .
The IR and Raman spectra of TMHD and D2-TMHD were also clearly assigned. Comparing the calculated and experimental
band frequencies and intensities suggest coexisting of two stable cis-enol conformers in comparable proportions in the sample.
The FT-IR spectrum of TMHD shows a very broad absorption that centered at about 2634 cm-1, for OH. By comparison of this
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