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)