spectroscopy labs
TRANSCRIPT
ABSTRACT:
The objective of this experiment is to demonstrate how computational chemistry
can be utilized to calculate vibrational frequencies and intensities of alkenes. The
accuracy of this technique is measured by comparing these results with data obtained
experimentally, however, a scaling factor must be considered in order to adjust the
computational values. An average scaling factor of 0.90 has been determined in our
results, thus, demonstrating how the computer interface overestimates the values by
approximately 10%. The computer interface, WebMO, does provide a list of all the
possible absorptions of the molecules being analyzed, however, not all these absorption
bands are observed in the simulated spectra.
INTRODUCTION:
Quantum mechanics is used to compute infrared spectral data which provides an
alternative method to obtain chemical properties of compounds without using any
expensive instruments or attending a laboratory for experimental processing. WebMO is
the web-based interface used which performs the computational chemistry calculations
for you from the comfort of using your home computer. All that is required from you is
for you to build the three-dimensional molecular structure of the compound on the
interface, and the program will do the rest of the work for you in simply a few seconds.
The C=C bond of ethene is analyzed by observing variational frequencies and
intensities in computationally generated infrared spectra of various monosubstituted
ethenes and how they relate to the experimental data. It is known, however, that there is
an approximate 10% overestimate of computational wavenumbers calculated due to
systematic errors.1
This overestimate in figures generally range between 0.8 and 1.0,
depending on the compound and the quality of the calculation.2
This means that an
empirical factor must be taken into account when converting computational frequencies
to experimental data and vice versa.
An advantage in using computational chemistry to calculate vibrational
frequencies with the WebMO interface is that low intensity and even inactive vibrations
can be animated, and located in the computed spectrum.3
This allows for a superior
analysis of the molecule being studied with the determination of theoretical vibrational
frequencies which may not be present in experimental spectral data.
RESULTS:
VIBRATIONAL DATA FOR ETHENE AND MONOSUBSTITUTED ALKENES
COMPOUNDS
CALCULATED υ (C=C) EXPERIMENTAL υ (C=C)
υ (cm-1) INTENSITY υ (cm-1)BAND
INTENSITY
SCALING FACTOR
Ethene
1828.7779 Zero 1620 Zero 0.89
Propene
1806.6002 Weak 1650 Weak 0.91
Vinyl Alcohol
1830.6733 Strong 1660 Medium 0.91
Vinyl Amine
1846.9062 Strong 1670 Medium 0.90
THEORETICALLY DETERMINED EXPERIMENTAL C=C BAND OF ISO-BUTENE AND 2-
BUTENE
C=C BAND υ (cm-1)
COMPOUNDS:
Iso-Butene 2-Butene
COMPUTATIONA
L1880.39 1890.22
EXPERIMENTAL 1692.35 1701.20
Average Scaling Factor Used = 0.90
INFRARED SPECTRA OF COMPOUNDS:
ETHENE:
PROPENE:
C=C
Weak
VINYL ALCOHOL:
VINYL AMINE:
C=C
Strong
C=C
Strong
DISCUSSION:
Ethene and its monosubstituted derivatives; propene, vinyl alcohol, and vinyl
amine, are analyzed by determining vibrational frequencies and intensities of the C=C
band by using computational chemistry and comparing these values with experimental
data. Comparison with the experimental figures provided enabled the determination of
the scaling factor to further have an indication of the accuracy of using computational
chemistry to obtain spectral data. As mentioned previously, there is an approximate 10%
overestimate when computationally calculating vibrational frequencies, therefore, a 10%
discrepancy in results is expected between computational and experimental data. Using
the average scaling factor obtained from the four ethenes, the theoretically determined
experimental C=C band stretch of iso-butene and 2-butene is made possible.
By observing the experimental data presented, a trend is observed where the
addition of an alkyl group on ethene increases the vibrational frequency of the C=C
stretch. The more substituted the alkene, the more stable it is. Ethene has a experimental
C=C stretch of 1620 cm-1
. Substituting an H atom in ethene with a methyl, alcohol, and
an amine group in propene, vinyl alcohol, and vinyl amine, increases the vibrational
frequency to 1650, 1660, and 1670 cm-1
, respectively. This can be hypothesized by the
effect of having stronger C=C bonds due to electron delocalization and resonance effects
by the presence of monosubstituted groups on ethene. Having a methyl group on ethene
delocalizes electrons from the double bond by resonance effects resulting in a stronger
bond. Vinyl alcohol and vinyl amine have -OH and -NH2 functional groups substituted
on the ethene which further stabilizes this molecule by withdrawing electrons by
resonance from the C=C bond. With the increasing electronegativity of the substituents,
the wavenumber increases. As oxygen and nitrogen are both electronegative, they will
both inductively withdraw electrons from the C=C bond further providing an increased
stabilization of the molecule resulting in an increased vibrational frequency, therefore,
requiring more energy to break the bonds. The resonance form of C=N in vinyl amine
will have a higher wavenumber than the resonance form of C=O in vinyl alcohol since
C=O has a higher reduced mass which lowers the frequency and the wavenumber of the
molecule. In reality, the vinyl amine has a lower wavenumber than the vinyl alcohol,
since the nitrogen has a greater tendency to share its lone pair of electrons, thus reducing
the double bond effect of the ethene and decreasing its absorption wavelength. This
demonstrates how the computational method may not necessarily be ideal in calculating
vibrational data.
The theoretically determined experimental C=C stretch of iso-butene and
2-butene have been calculated to have wavenumbers of 1692.35 and 1701.20 cm-1
,
respectively. When comparing these values to the C=C stretch of 1650 cm-1 of propene,
the increase in frequencies are observed. This can be accounted by the fact that
iso-butene and 2-butene both have an additional methyl group (disubstituted), when
compared to propene, thus, providing even more stabilization by inductive and resonance
effects. Additional cis-trans isomers delocalize the electrons even more strengthening the
C=C bond and increasing the vibrational frequencies of the bond. Increased stabilization
in 2-butene is resultant from the structural internal symmetry of the C=C bond, leading to
decreased polarization and a higher energy required to break the bond (1701.20 cm-1)
. Iso-
butene contains germinal methyl groups increasing polarity requiring slightly less energy
to break the C=C bond (1692.35 cm-1
).
When ethene is substituted with a ethyl group, a band stretch is obtained at 1650
cm-1
, however, when it is substituted with a vinyl group, a decreased band stretch at 1600
cm-1
is obtained. It is known that with additional alkyl groups the C=C bond is further
stabilized resulting in an increase in vibrational frequency. However, in the case of
ethene substituted with a vinyl group, a decreased frequency is observed. This is
explained by the fact that conjugation present in the ethene substituted with a vinyl group
will increase the single bond characteristic of the C=C bond, thus, weakening the bond
and resulting in a decreased frequency.
The vibrational frequency of the C=C stretch of ethene is computationally
calculated to an overestimated value of 1828.7779 cm-1 having a scaling factor of 0.89,
when compared to the experimental stretch of 1620 cm-1
. The vibrational frequency of the
C=C stretch of propene is computationally calculated to an overestimated value of
1806.6002 cm-1 having a scaling factor of 0.91, when compared to the experimental
stretch of 1650 cm-1
. The vibrational frequency of the C=C stretch of vinyl alcohol is
computationally calculated to an overestimated value of 1830.6733 cm-1 having a scaling
factor of 0.91, when compared to the experimental stretch of 1660 cm-1
. The vibrational
frequency of the C=C stretch of vinyl amine is computationally calculated to an
overestimated value of 1846.9062 cm-1 having a scaling factor of 0.90, when compared to
the experimental stretch of 1670 cm-1
. The scaling factors between the experimental and
computational data have averaged to a scaling factor of 0.90. This further confirms the
fact that computational chemistry overestimates wavenumbers by approximately 10%.
Computational chemistry accurately approximates band stretches, however, an empirical
factor must be accounted to obtain corrected experimental band stretches.
The absorption within 1000 and 800 cm-1
are referred to as the fingerprint region
where out of plane bending vibrations occur, such as twisting and wagging. For non-
linear molecules with N atoms, 3N-6 amount of vibrations are expected. For linear
molecules with N atoms, 3N-5 amount of vibrations are expected. Ethene has 6 atoms,
therefore, 3(6)-6 = 12 stretching vibrations are expected.4
In the simulated spectrum only
2 vibrations are observed. Vinyl alcohol has 7 atoms and (3(7)-6 = 15 amount of
vibrations are expected, however, not all these absorptions are observed in the simulated
spectrum. For ethene, the peak corresponding to -CH stretch is observed and for vinyl
alcohol the peaks observed were the C=C, -C-H, -OH, and -C-O. As ethene is a
symmetrical compound, only one absorptions id observed in the simulated IR spectrum.
All the vibrations occurring in the symmetric molecule are cancelling each other out.
Propene contains an additional methyl group as one of the substituents on ethene, which
is not creating a large enough dipole moment for absorption to occur, and can be
observed to have only one peak as well on the simulated spectrum. Due to the fact that
vinyl alcohol and vinyl amine both contain electronegative atoms, oxygen and nitrogen,
presence of polarization in the molecule causes it to have more absorption in the IR
spectrum.
CONCLUSION:
Computational chemistry enabled the calculation estimations of vibrational
frequencies and intensities, however, a 10% overestimation is present in the calculated
values which are required to be adjusted in order to be comparable to the experimental
absorptions. This method also does not consider interactions of the molecules at the
molecular level, as oxygen and nitrogen are both electronegative atoms, but they also
have the ability to share their lone par of electrons inductively which also influence the
absorption frequencies and are atomic characteristics that must be accounted when
applying computational chemistry to obtain data. The IR spectra simulated do generate a
list of all the absorptions possible within a molecule, however, not all the absorption
bands are present in the simulated spectra, and we have seen how structural symmetry
decreases the amount of absorption bands present in the simulated spectra due to the lack
of polarization of the molecule. Computational chemistry may provide absorption
estimations that must be scaled before accounting them as acceptable values, however, to
obtain accurate analysis and data of absorptions frequencies, perhaps utilizing an IR
instrument and performing experimental chemistry may be the best option available for
spectral analysis.
REFERENCES:
1, 2, 3
Laboratory Manual, Chem 393 Spectroscopy and Structure of Organic Compounds,
Prepared by Dr. T.J. Adley and F. Nudo, Modified by Dr. H.M. Muchall, Department of
Chemistry and Biochemistry, Concordia University. (pp.11-13)
4
Introduction to Spectroscopy, Pavia, Lampman, Kriz, Third Edition, Washington, 2001.
(p. 68-75)