Internal Rotation of Methylcyclopropane and Related Molecules

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Internal Rotation of Methylcyclopropane and Related Molecules: Comparison of Experiment and Theory Esther J. Ocola, and Jaan Laane J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b06783 • Publication Date (Web): 29 Aug 2016 Downloaded from http://pubs.acs.org on August 29, 2016

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The Journal of Physical Chemistry

Internal Rotation of Methylcyclopropane and Related Molecules: Comparison of Experiment and Theory Esther J. Ocola and Jaan Laane* Department of Chemistry, Texas A&M University, College Station, TX 77843-3255, USA

Corresponding Author, Email address: [email protected], Phone: 979-845-3352 1 ACS Paragon Plus Environment


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ABSTRACT

The internal rotation about the single bond connecting a cyclopropyl ring to a CH3, SiH3, GeH3, NH2, SH, or OH group has been investigated. Both CCSD/cc-pVTZ and MP2/cc-pVTZ ab initio calculations have been carried out to predict the structures of these molecules and their internal rotation potential energy functions in terms of angles of rotation. The barriers to internal rotation for the CH3, SiH3, GeH3 molecules from the calculations agree well with the experimental ones, within -11% to +1% for CCSD/cc-pVTZ and -4% to +9% for MP2/cc-pVTZ. Comparisons between theory and experiment were also carried out for propylene oxide and propylene sulfide and the agreements were very good. Theoretical calculations were carried out to compute the internal rotation potential energy function for cyclopropanol and these were used to guide the determination of a potential function based on experimental data.

This molecule has two

equivalent synclinal (gauche) conformers with an estimated barrier of 759 cm-1 (9.1 kJ/mol) between them. The minima are at internal rotation angles of the OH group of 109º and 251º. The theoretical potential functions for cyclopropanethiol and cyclopropylamine were also calculated and these agree reasonably well with previous experimental studies.

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INTRODUCTION The investigations of internal rotations about single bonds using far-infrared, microwave, and Raman spectroscopy and theoretical calculations have been of interest for more than fifty years.1-19 Among the molecules which have been studied are those where a cyclopropyl ring is attached to the single bond. In our laboratory we have experimentally determined the barriers to rotation of methylcyclopropane (CP-CH3)20, propylene oxide (CPO-CH3)20, and propylene sulfide (CPS-CH3)20 by Raman spectroscopy and cyclopropylsilane (CP-SiH3)21,22 and cyclopropylgermane (CP-GeH3)22 by mid-infrared combination bands. In the present study we have carried out high level ab initio calculations for each of these molecules and also for cyclopropanol (CP-OH) and cyclopropanethiol (CP-SH) in order to assess how well the theoretical calculations do for predicting the barriers and potential energy functions. Experimental microwave results have been previously reported for CP-OH23 and CP-SH24, and low level theoretical calculations were also performed on CP-OH25. The far-infrared spectra of cyclopropylamine (CP-NH2) has previously been studied by Kalasinsky and co-workers26 and a potential energy function was determined. CH3

CH3

SiH3

CPS-CH3

CP-SiH3

S

O CP-CH3

CH3

CPO-CH3

GeH3

OH

CP-GeH3

CP-OH

SH

CP-SH


NH2

CP-NH2


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COMPUTATIONS

Structure Calculations. The cc-pVTZ (triple-ζ) basis set was used with the CCSD and the second-order Møller-Plesset (MP2) ab initio methods to calculate the structures and conformational energies for each of the molecules. The GAUSSIAN 09 program27 was used for the computations and the Semichem AMPAC/AGUI program28 was used to visualize the structures. For each of the molecules except CP-NH2 the origin of the internal rotational angle, ф, was defined such that the hydrogen atom of the rotor would be pointing directly at the midpoint of the C-C (or C-O or C-S) bond across the ring from it. For CP-NH2 the origin of ф was taken to be zero where the midpoint between the two hydrogen atoms was directed at the C-C bond across from it.

Theoretical potential energy functions. The conformational energies for several dozen values of the internal rotation angle ф were calculated using both CCSD/cc-pVTZ and MP2/cc-pVTZ basis sets.

These were then fitted to the following function with the aid of the

MAPLE 2015.1 computing environment29 in order to determine the values of the potential energy constants Vn: (1)

In cases were the function produced negative values of the potential energy V a positive term was added in order to adjust the energy minimum to be zero.

Energy level calculations.

The energy levels and wavefunctions for the periodic

potential energy functions were calculated using our VNCOSPX program.30 4 ACS Paragon Plus Environment

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RESULTS AND DISCUSSION

Structures.

Figure 1 presents the calculated CCSD/cc-pVTZ structures for each of the

molecules showing the conformations possessing energy minima.

For the molecules CP-CH3,

CPO-CH3, CPS-CH3, CP-SiCH3, and CP-GeH3 each molecule has three equivalent minimum energy conformations.

Molecules CP-OH and CP-SH each have two equivalent synclinal

(gauche) conformers. The calculations also predict a third very shallow minimum for the trans structures but these have not been observed experimentally. Table 1 compares the calculated geometrical parameters to the experimental microwave or electron diffraction values where available. As can be seen, the agreement is excellent. Table 2 compares the rotational constants for our calculated structures to experimental ones for the molecules for which microwave data are available.

Again the agreement is very good. For the molecules CP-CH3, CP-SiH3, and

CP-GeH3 it can be seen that the carbon-carbon bond distances in the rings are affected by a small amount by the substituents. The electropositive silicon or germanium substituent weakens and lengthens the bonds from the carbon to which the MH3 (M = C, Si, or Ge) group is attached while the bond opposite is shortened. Attaching an electronegative OH or NH2 group has the opposite effect. The slightly electronegative SH group has a smaller effect. It is also interesting that the internal rotation angle of the OH or NH2 groups lengthens the adjacent carbon-carbon bond on one side of the ring while decreasing the other side.



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Figure 1. Calculated CCSD/cc-pVTZ structures for each of the molecules showing the conformations possessing energy minima.


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1Table 1. Comparison of experimental and calculated geometrical parameters for methylcyclopropane and analogue molecules 2 3 4 5 CP-OH CP-SH CP-CH3 CPO-CH3 CPS-CH3 CP-SiH3 CP-GeH3 6 7 8 9 10 11 EDa Calc.b MWc Calc.b EDd Calc.b EDe Calc.b EDf Calc.b MWg Calc.b Calc.h Calc.b 12 13 14Bond distances (Å) 1.509 1.504 1.435 1.426 1.831 1.827 1.528 1.517 1.521 1.513 --1.501 1.505 15 C1-(C2, O, S) 16 (1.503) (1.435) (1.827) (1.517) (1.513) (1.500) 1.505 (1.504) 17 1.509 1.504 1.470 1.464 1.495 1.481 1.528 1.517 1.521 1.513 --1.490 1.506 C1-C3 18 (1.503) (1.462) (1.479) (1.517) (1.513) (1.488) 1.506 (1.506) 19 j 1.510 1.441 1.427 1.831 1.824 1.490 1.498 1.502 1.501 1.52 1.516 1.505 1.510 20 (C2, O, S)-C3 (1.509) (1.436) (1.823) (1.497) (1.500) (1.515) 1.505 (1.504) 21 22 C (C ,O,Si,Ge,N) 1.517 1.508 1.505 1.502 1.502 1.507 1.840 1.863 1.926 1.39 1.399 1.791 1- 4 23 (1.504) (1.497) (1.503) (1.858) 1.924 (1.920) (1.400) 1.784 (1.783) 24 25 Angles (degrees) 26 C -(C , O, S)-C 60.0 59.9 61.5 61.7 48.2 47.9 60.8 60.4 60.4 60.3 --59.2 60.1 1 2 3 27 (59.9) (61.2) (47.8) (60.4) (60.3) (59.1) 60.0 (60.1) 28 60.0 60.3 59.4 59.2 65.9 66.0 58.4 59.1 59.2 59.4 --60.9 60.0 29 (C2, O, S)-C1-C3 (60.3) (59.4) (66.0) (59.1) (59.4) (60.9) 60.0 (59.9) 30 31 (C2, O, S)-C3-C1 60.0 59.9 59.1 59.1 65.9 66.2 60.8 60.4 60.4 60.3 --59.9 60.0 32 (59.9) (59.3) (66.2) (60.4) (60.3) (59.9) 60.0 (60.0) 33 a Ref 31. 34 b This work. CCSD/cc-pVTZ values top row. MP2/cc-pVTZ values bottom row in parenthesis and in italics. 35 c Ref 32. 36 d Ref 33. 37 e Ref 34. 38 f Ref 35. 39 g Ref 23. 40 h MP2/aug-cc-pVTZ values Ref 24. 41 i Ref 37. 42 j 43MW, Ref 37. 44 7 45 46 ACS Paragon Plus Environment 47 48

CP-NH2

MWi

Calc.b

1.498

1.501 (1.500) 1.501 (1.500) 1.510 (1.509) 1.442 (1.439)

1.498 1.514 1.451

59.7 60.7 59.7

59.8 (59.8) 60.4 (60.4) 59.8 (59.8)


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Molecules with methyl rotors. We have previously reported the vapor-phase Raman spectra and internal rotation potential energy functions for molecules CP-CH3, CPO-CH3, and CPS-CH3. The potential functions have the form of eq. (1) above where the Vn are potential energy parameters and ф is the angle of internal rotation of the methyl group.

For these

molecules the internal rotation is represented only by V3 and V6 terms where V3 is the dominant term and determines the barrier to internal rotation. The V6 term primarily serves as a shaping parameter and is of much smaller magnitude. In our present work we have calculated point by point values of the potential energy V and then used mathematical software to determine the values of V3 and V6. Table 3 compares these theoretical values for CP-CH3, CPO-CH3, and CPS-CH3 to those determined experimentally.20

It also shows the comparisons for molecules

CP-SiH3 and CP-GeH3 which will be discussed below.

Figures 2 to 4 show both the

experimental and theoretical potential energy functions for these molecules. They also show the observed Raman frequencies for the internal rotation transitions.

In all cases it is evident that

both types of theoretical calculations agree remarkably well with the experimental results.

It

should also be kept in mind that the experimental values are not perfect and just a best fit for about three observed transitions so they will have some uncertainties.

Table 2. Comparison of experimental and theoretical rotational constants

A

Theoreticala, MHz A B

Ref

C

CPO-CH3

18023.87

6682.14

5951.39

39

18235.57

6666.60

5939.67

CP-OH

16664.98

6911.28

5955.04

23

16836.00

6903.20

5951.31

CP-SH

15702.37

3894.72

3585.17

24

15885.16

3833.78

3531.03

16429.03

6532.05

5658.15

CP-NH2 a

Experimental, MHz B C

16245.77

6538.51

5660.66

37

b

B3LYP/cc-pVTZ calculation. Measured for 15N.

b


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Table 3. Comparison of potential energy parameters for the internal rotation of MHn groups attached to three-membered rings (cm-1) Molecule

CP-CH3 CPO-CH3 CPS-CH3 CP-SiCH3 CP-GeCH3

Experimental F

V3

V6

Ref

5.755 5.841 5.604 3.378 3.249

1060 893 1148 694 474

9 -7 -13 10 0

20 20 20 21 22

Theoretical CCSD/cc-pVTZ MP2/cc-pVTZ V3 V6 V3 V6 987 900 1121 616 472

9 -37 -42 -18 -4

1030 936 1180 662 517

9 -38 -45 -10 -4

Figure 2. Experimental and theoretical internal rotation potential energy functions for methylcyclopropane (CP-CH3). The experimentally observed Raman transitions20 are also shown in the figure.



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Figure 3. Experimental and theoretical internal rotation potential energy functions for propylene oxide (CPO-CH3). The experimentally observed Raman transitions20 are also shown in the figure.


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Figure 4. Experimental and theoretical internal rotation potential energy functions for propylene sulfide (CPS-CH3). The experimentally observed Raman transitions20 are also shown in the figure.

Cyclopropylsilane and cyclopropylgermane. These two molecules also have potential energy functions of the type in eq. (1) for the internal rotation of the MH3 groups. These were determined experimentally from mid-infrared combination bands.21,22 The comparison between experiment and theory is shown in

Table 3, and Figures 5 and 6 compare the theoretical and

experimental potential energy curves. The agreement is again very good. As expected, the barrier to internal rotation of 1060 cm-1 for the methyl group drops to 694 cm-1 for the SiH3 and 474 cm-1 for the germane.



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Figure 5. Experimental and theoretical internal rotation potential energy functions for cyclopropylsilane (CP-SiH3). The experimentally observed infrared transitions21 are also shown in the figure.


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Figure 6. Experimental and theoretical internal rotation potential energy functions for cyclopropylgermane (CP-GeH3). The experimentally observed infrared transitions22 are also shown in the figure.

Cyclopropanol and cyclopropanethiol.

For these two molecules as well as for

cyclopropylamine to be discussed later the discussion of specific conformational minima and barriers can become confusing. Therefore, in Table 4 we present a summary of calculated conformational energies and internal rotation angles corresponding to the various minima and barriers. These will be specifically discussed below. MacDonald and co-workers23 investigated the microwave spectrum of cyclopropanol and found the synclinal (gauche) conformations to be lowest in energy. They reported the energy minimum to be 106º ± 5º and estimated the cis barrier to be 7.9 kJ/mol

(660 cm-1) and the trans

barrier to be about 17 kJ/mol (1420 cm-1). In a later work25 they reported early ab initio 13 ACS Paragon Plus Environment


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Table 4. Minima and barriers of CP-OH, CP-SH and CP-NH2 Molecule

Source Energy (cm-1)

ф

a

Minima Energy (cm-1)

ф

a

Energy (cm-1)

ф

a

Barriers Energy (cm-1)

ф

a

CP-OH gauche

trans

trans

cis

CCSD/cc-pVTZ

0

109.1°, 250.9°

758

0.0°

842

180.0°

765

-22.0°, 22.0°

MP2/cc-pVTZ

0

109.1°, 250.9°

764

0.0°

864

180.0°

775

-23.0°, 23.0°

Refined

0

109.1°, 250.9°

671

0.0°

759

180.0°

682

-24.0°, 24.0°

CP-SH gauche

trans

trans

cis

CCSD/cc-pVTZ

0

108.7°, 251.3°

834

0.0°

1189

180.0°

873

-27.0°, 27.0°

MP2/cc-pVTZ

0

108.7°, 251.3°

884

0.0°

1284

180.0°

927

-25.0°, 25.0°

CP-NH2 trans

trans

gauche

CCSD/cc-pVTZ

0

0.0°

753

132.4°, 227.6°

1507

-72.5°, 72.5°

956

180.0°

MP2/cc-pVTZ

0

0.0°

777

132.4°, 227.6°

1565

-72.5°, 72.5°

981

180.0°

0

0.0°

592

133.6°, 226.4°

1266

-66.7°, 66.7°

1007

180.0°

b

Experimental a

gauche

Calculated angle of internal rotation for the indicated conformation. Ref. 26.

b


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Figure 7. Theoretical and refined internal rotation potential energy functions for cyclopropanol (CP-OH). The experimentally observed infrared transitions23,25 are also shown in the figure. The frequencies in parentheses correspond to calculated values.

calculations using a 4-21G basis set. These show a potential energy function where the barrier between the two gauche configurations (cis barrier) is about 900 cm-1 (10.8 kJ/mol) and the barrier between the trans and gauche configurations (trans barrier) is about 700 cm-1 (8.4 kJ/mol). In the present work we have calculated the potential energy function and this is shown in Figure 7 for both CCSD/cc-pVTZ and MP2/cc-pVTZ computations. As shown in Figures 1 and 7, this molecule is calculated to also have a very shallow minimum of a few cm-1 corresponding to the trans structure but no evidence for this has been observed. From the CCSD/cc-pVTZ calculation the trans form is calculated to be 758 cm-1 (9.07 kJ/mol), and from MP2/cc-pVTZ is calculated to be 764 cm-1 (9.14 kJ/mol) higher in energy than the synclinal



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minima.

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The synclinal conformers have a calculated CCSD/cc-pVTZ and MP2/cc-pVTZ

internal rotation angles of the OH group of 109.1° and 250.9° degrees. calculated to be 842 cm-1 (10.07 kJ/mol) MP2/cc-pVTZ.

from

CCSD/cc-pVTZ

The cis barrier is and 864 cm-1 from

The trans barrier is calculated to be 765 cm-1 (9.15 kJ/mol)

from

CCSD/cc-pVTZ and 775 cm-1 (9.27 kJ/mol) from MP2/cc-pVTZ The MacDonald group23,25 determined the ground state splitting to be 4115.26 MHz (0.137 cm-1) and the 0 to 1 spacing to be 264.2 cm-1. Based on these values and their low level ab initio calculations they presented partial representations of possible potential functions. In our present work we have utilized these experimental results to refine the results of our theoretical calculations and have obtained a potential energy function which predicts the experimental results and also is compatible with the theoretical work. This has a barrier of 759 cm-1 (9.1 kJ/mol) between the synclinal forms and minima at 109.1° and 250.9° degrees in agreement with the theoretical calculations. The trans configuration is calculated to be 671 cm-1 (8.0 kJ/mol) higher in energy of the gauche form and the trans barrier 682 cm-1 (8.2 kJ/mol). Figure 7 shows the potential energy curves from the CCSD/cc-pVTZ and MP2/cc-pVTZ calculations along with our refined potential which fits the experimental data.

Table 5 compares the potential energy parameters Vn for the

CCSD/cc-pVTZ, MP2/cc-pVTZ and the refined functions.

Table 6 compares the calculated

transition frequencies for the three functions and it can be seen that the refined function fits the experimental data very well. The Møllendal group24 has reported the microwave spectrum of cyclopropanethiol (CP-SH) and found that the molecule has two equivalent synclinal conformers.

They also

reported the ground state torsional energy level splitting to be 1.664 MHz. They also carried out a B3LYP/aug-cc-pVTZ calculation which predicted a second energy minimum at 9.32 kJ/mol


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(779 cm-1), and two potential energy barriers barriers of 9.61 kJ/mol (803 cm-1) and 12.50 kJ/mol (1045 cm-1). Our calculations for this molecule are similar to those for the cyclopropanol in that the synclinal has the lowest energy. The synclinal conformers have a calculated CCSD/cc-pVTZ and MP2/cc-pVTZ internal rotation angles of the SH group of 108.7° and 251.3° degrees. As shown in Figure 8, the calculated CCSD/cc-pVTZ cis barrier is 1189 cm-1 (14.22 kJ/mol) and the calculated MP2/cc-pVTZ cis barrier is 1284 cm-1 (15.36 kJ/mol).

The calculated

CCSD/cc-pVTZ trans structure is 834 cm-1 (9.98 kJ/mol) and the MP2/cc-pVTZ calculated structure is 884 cm-1 (10.57 kJ/mol). The calculated CCSD/cc-pVTZ trans barrier is 873 cm-1 (10.44 kJ/mol) and the MP2/cc-pVTZ calculated trans barrier is 927 cm-1 (11.09 kJ/mol). Table 5 shows the calculated Vn terms for both of the theoretical calculations.

Again a very

shallow minimum is calculated for the trans structure, but if present, this would not be deep enough to allow the identification of this conformer.

Figure 8 shows the calculated infrared

transition frequencies for the lower levels of this molecule.

Table 5. Comparison of potential energy parameters Vn F CP-OH

Source a

21.94

b

CCSD/cc-pVTZ

b

MP2/cc-pVTZ Refined

CP-SH

b

12.8

b b

CCSD/cc-pVTZ

b

MP2/cc-pVTZ

CP-NH2

c

9.80

b

CCSD/cc-pVTZ

b

MP2/cc-pVTZ

c

Experimental

V1

V2

V3

V4

V5

V6

-288

-685

356

5

17

-6

-287

-689

374

3

14

0

-253

-606

329

3

12

-1

-174

-921

455

-2

74

16

-188

-950

522

-30

62

0

301

859

706

-25

-60

-9

313

892

734

-26

-63

-9

280

576

728

0

0

-80

a

Ref 25. This work. c Ref 26. b



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a

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b

Table 6. Observed and calculated transition frequencies (cm-1) for cyclopropanol Transition

Observed

+

-

-

+

0 →0 0 →1 +

-

+

-

0 →1 1 →1

0.137

{

CCSD/cc-pVTZ 0.084

Calculated MP2/cc-pVTZ 0.086

Refinedc 0.137

278.8

285.3

264.1

280.8

287.2

266.9

1.9

1.8

2.7

264.2 1.9

a

Ref 25. Transition frequencies were calculated using the VNCOSPX program using the Vn values given in Table 5. c Calculated using the refined Vn values which were adjusted in the present work to fit the data.

b

Figure 8. Theoretical internal rotation potential energy functions for cyclopropanethiol (CP-SH). The calculated values from the CCSD/cc-pVTZ computation are shown. The vibrational frequencies shown are from using the potential energy function calculated from the CCSD/cc-pVTZ computation and the output of the VNCOSPX program. 18 ACS Paragon Plus Environment

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Cyclopropylamine.

Cyclopropylamine (CP-NH2) possesses two different barriers to

interconversion as shown in Figure 9. Kalasinsky et al.26 reported the experimental barriers to internal rotation of the NH2 group of this molecule to be 1007 ± 150 cm-1 and 1266 ± 150 cm-1. They also reported a second energy minimum (gauche conformer) at 592 cm-1.

Their

experimental potential energy curve determined internal rotation angles of 133.6° and 226.4° degrees of the NH2 group for the gauche conformers.

In the present work the calculated

CCSD/cc-pVTZ gauche conformer is 753 cm-1 (9.00 kJ/mol) and the calculated MP2/cc-pVTZ gauche conformer is 777 cm-1 (9.29 kJ/mol) higher in energy than the trans minima. The gauche conformers have calculated CCSD/cc-pVTZ and MP2/cc-pVTZ internal rotation angles of the NH2 group of 132.4° and 227.6° degrees. As shown in Figure 9, the calculated CCSD/cc-pVTZ trans barrier is 1507 cm-1 (18.03 kJ/mol) and the calculated MP2/cc-pVTZ trans barrier is 1565 cm-1 (18.72 kJ/mol).

The calculated CCSD/cc-pVTZ gauche barrier is 956 cm-1

(11.44 kJ/mol) and the MP2/cc-pVTZ calculated gauche barrier is 981 cm-1 (11.74 kJ/mol). Table 5 compares the potential energy parameters from our CCSD/cc-pVTZ and MP2/cc-pVTZ calculations to those from the experimental study. Table 7 compares the observed infrared and predicted transition frequencies based on the theoretical ab initio calculations and using the VNCOSPX program. Figure 9 shows our calculated potential energy function and compares it to that previously reported.

The agreement is quite reasonable. However, the lower calculated -

barrier between the two gauche conformers gives rise to a much lower 0 → 1+ transition frequency of 123.8 cm-1 (CCSD/cc-pVTZ) than that observed at 176 cm-1. The ab initio calculations for this molecule in its trans conformer based on the harmonic model predict the NH2 torsion to be at 263 cm-1 (DFT/B3LYP/cc-pVTZ calculation scaled by 0.985) and this agrees well with the assignment of 254 cm-1. For the gauche structure the calculated frequency 19 ACS Paragon Plus Environment


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Figure 9. Experimental and theoretical internal rotation potential energy functions for cyclopropylamine (CP-NH2). The experimentally observed infrared transitions26 are also shown in the figure.

a

b

Table 7. Observed and calculated transition frequencies (cm-1) for cyclopropylamine Transition Obs.

Trans Frequencies Calculated CCSD/cc-pVTZ

0→1 0→2 1→3 2→4 a b

254 500 482 452

270.3 530.7 507.3 475.4

Transition

MP2/cc-pVTZ +

275.7 541.6 518.5 487.2

-

0 →1 0 → 1+

Gauche Frequencies Obs. Calculated 188 176

CCSD/cc-pVTZ

MP2/cc-pVTZ

188.7 123.8

191.1 127.3

Ref 26. Transition frequencies were calculated using the VNCOSPX program using the Vn values given in Table 5.


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Figure 10. Calculated wavefunctions for CP-NH2 for the experimental energy function from Ref 26. 21 ACS Paragon Plus Environment


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is 194 cm-1 which can be compared to the experimental doublet at 188/176 cm-1. Figure 10 shows the wavefunctions for the experimental potential energy curve calculated using the VNCOSPX program. It can clearly be seen which energy levels belong to the trans structure and which to the gauche.

CONCLUSIONS We have carried out detailed theoretical ab initio CCSD/cc-pVTZ and MP2/cc-pVTZ calculations to predict the potential energy functions of the internal rotation of seven molecules containing three-membered rings.

The five molecules with MH3 rotors (M = C, Si, Ge) have

simple three-fold potential functions and the theoretical calculations do a very good job of predicting the barriers.

For CP-OH the theoretical calculations provided potential energy

parameters which with relatively small refinements we were able to reproduce the experimental results for the first time. The barriers for the CP-SH molecule were found to be greater than for the CP-OH.

For CP-NH2 the theoretical calculation confirms the existence of both trans and

gauche conformers although the calculated energy difference experimentally is about 150 cm-1 less than that from the calculation.

The calculations also predict a smaller energy barrier

between the two gauche conformers.

ACKNOWLEDGEMENTS The authors wish to thank the Robert A. Welch Foundation (Grant A-0396) for financial support. Computations were carried out on the Texas A&M University Department of Chemistry Medusa computer system funded by the National Science Foundation, Grant No. CHE-0541587. The Laboratory for Molecular Simulation provided the Semichem AMPAC/AGUI software.


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REFERENCES (1) Fateley, W. G.; Miller, F. A. Torsional Frequencies in the Far Infrared - I. Molecules with a Single Methyl Rotor. Spectrochim. Acta 1961, 17, 857-868. (2) Fateley, W. G.; Miller, F. A. Torsional Frequencies in the Far Infrared-III. The Form of the Potential Curve for Hindered Internal Rotation of a Methyl Group. Spectrochim. Acta 1963, 19, 611-628. (3) Dreizler, H.

Microwave Spectroscopic Determination of Rotational Barriers of Free

Molecules. Fortschr. Chem. Forsch. 1968, 10, 59-155. (4) Lowe, J. P. Barriers to Internal Rotation about Single Bonds. Prog. Phys. Org. Chem. 1968, 6, 1-80. (5) Pethrick, R. A.; Wyn-Jones, E. The Determination of the Energies Associated with Internal Rotation. Q. Rev. Chem. Soc. 1969, 23, 301-324. (6) Fateley, W. G.; Manocha, A. S.; Tuazon, E. Barrier and Conformations. II. Far-Infrared Investigation of Internal Rotation about Single Bonds. Pure Appl. Chem. 1973, 36, 119-126. (7) Lowe, J. P. The Barrier to Internal Rotation in Ethane. Science 1973, 179, 527-532. (8) Durig, J. R. Determination of Barriers to Internal Rotation around Single Bonds.

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Vibrational Spectroscopy: Modern Trends; Barnes A. J.; Orville-Thomas, W. J. (Eds.) Elsevier Science Ltd.; Amsterdam: The Netherlands, 1977; p 321-334. (9) Lister, D. G.; MacDonald, J. N.; Owen, N. L.

Internal Rotation and Inversion: An

Introduction to Large Amplitude Motions in Molecules; Academic Press Inc.: London, 1978. (10) Groner, P.; Sullivan, J. F.; Durig, J. R. Internal Rotation of Molecules with Two C3v Rotors Vib. Spec. Struct. 1981, 9, 405-496.



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(11) Pitzer, R. M. The Barrier to Internal Rotation in Ethane. Accounts Chem. Res. 1983, 16, 207-210. (12) Long, D. A. Internal Rotation: A Historical Perspective. J. Mol. Struct. 1985, 126, 9-24. (13) Ito, M. Spectroscopy and Dynamics of Aromatic Molecules Having Large-Amplitude Motions. J. Phys. Chem. 1987, 91, 517-526. (14) Smeyers, Y. G.; Hernandez-Laguna, A. Non-Rigid Molecules. An Application to Internal Rotation. Stud. Phys. Theor. Chem. 1992, 77 (A), 407-433. (15) Spangler, L. H.; Pratt, D. W. Internal Rotation Dynamics from Electronic Spectroscopy in Supersonic Jets and Beams.

In Jet Spectroscopy and Molecular Dynamics; Hollas, J. H.;

Phillips, D. (Eds.); Springler Science + Business Media LCC; New York, 1995; 396-398. (16) Spangler, L. H. Structural Information from Methyl Internal Rotation Spectroscopy. Annu. Rev. Phys. Chem. 1997, 48, 481-510. (17) Kleiner, I.

Dynamics of One-Dimensional Large Amplitude Motions: Molecular

Hamiltonians. J. Chim. Phys. 1998, 95, 1831-1875. (18) Goodman, L.; Pophristic, V.; Weinhold, F. Origin of Methyl Internal Rotation Barriers. Accounts Chem. Res. 1999, 32, 983-993. (19) Kundu, T.; Pradhan, B.; Singh, B. P. Origin of Methyl Torsional Potential Barrier – An Overview. P. Indian Acad. Sci. (Chem. Sci.) 2002, 114, 623-638. (20) Villareal, J. R.; Laane, J. Raman Spectra and Internal Rotation of Methylcyclopropane and its Analogs. J. Chem. Phys. 1975, 62, 303-304. (21) Laane, J.; Nour, E. M.; Dakkouri, M. Barrier to Internal Rotation of the Silyl Group in Cyclopropylsilane from Combination Band Spectra. J. Mol. Spec. 1983, 102, 368-371.


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(22) Kelly, M. B.; Laane, J.; Dakkouri, M. Barriers to Internal Rotation of Cyclopropylsilane-d3 and Cyclopropylgermane from Combination Band Spectra. J. Mol. Spec. 1989, 137, 82-86. (23) Macdonald, J. N.; Norbury, D.; Sheridan, J. Microwave Spectrum, Dipole Moment and Internal Rotation Potential Function of gauche-Cyclopropanol. J. Chem. Soc., Faraday Trans. 2. 1978, 74, 1365-1375. (24) Mokso, R.; Møllendal, H.; Guillemin, J-C. A Microwave and Quantum Chemical Study of Cyclopropanethiol. J. Phys. Chem. A. 2008, 112, 4601-4607. (25) Plant, C.; Spencer, K.; Macdonald, J. N. The Conformations of Some Simple Alcohols and Thiols. J. Chem. Soc., Faraday Trans. 2. 1987, 83, 1411-1425. (26) Kalasinsky, V. F.; Powers, D. E.; Harris, W. C. Vibrational Spectra and Conformations of Cyclopropylamine. J. Phys. Chem. 1979, 83, 506-510. (27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09, Revision A.02, Gaussian, Inc.: Wallingford, CT, 2009. (28) AGUI program from Semichem, Inc.: Shawnee, KS 66216: www.semichem.com. (29) Maple 2015 program from Waterloo Maple Inc.: Waterloo, OH, Canada: www.maplesoft.com (30) Lewis, J. D.; Malloy, T. B. Jr.; Chao, T. H.; Laane, J. Periodic Potential Functions for Pseudorotation and Internal Rotation. J. Mol. Struct. 1972, 12, 427-449. (31) Klein, A. W.; Schrumpf, G. Molecular Structure of Methylcyclopropane and trans-1,2Dimethylcyclopropane as Studied by Gas Electron Diffraction. Acta Chem. Scand. A, 1981, 35, 425-430.



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(32) Imachi, M.; Kuczkowski, R. L. The Microwave Spectrum and Structure of Propylene Oxide. J. Mol. Struct. 1982, 96, 55-60. (33) Aarset, K.; Page, E. M.; Rice, D. A. The Molecular Structure of Propylene Sulphide (Methylthiirane) by Gas-Phase Electron Diffraction and Theoretical Calculations: A Molecule Used in MOCVD. J. Mol. Struct. 2011, 1002, 19-23. (34) Dakkouri, M.; Typke, V. Electron Diffraction Investigation of the Molecular Structure of Cyclopropyl Silane. J. Mol. Struct. 1987, 158, 323-337. (35) Dakkouri, M. Experimental and Theoretical Investigation of the Molecular Structure of Cyclopropylgermane. J. Am. Chem. Soc. 1991, 113, 7109-7114. (36) Rall, M.; Harmony, M. D.; Cassada, D. A.; Staley, S. W. Microwave Structure of Cyclopropylamine: Substituent Effect of the Amino Group. J. Am. Chem. Soc. 1986, 108, 61846189. (37) Penn, R. E.; Boggs, J. E. Substituent-induced Asymmetry of the Cyclopropane Ring. J. Chem. Soc., Chem. Commun. 1972, 11, 666-667. (38) Creswell, R. A.; Schwendeman, R. H. Centrifugal Distortion Constants and Structural Parameters of Methyl Oxirane. J. Mol. Spec. 1977, 64, 295-301.


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Figure Captions Figure 1. Calculated CCSD/cc-pVTZ structures for each of the molecules showing the conformations possessing energy minima. Figure 2. Experimental and theoretical internal rotation potential energy functions for methylcyclopropane (CP-CH3). The experimentally observed Raman transitions20 are also shown in the figure. Figure 3. Experimental and theoretical internal rotation potential energy functions for propylene oxide (CPO-CH3). The experimentally observed Raman transitions20 are also shown in the figure. Figure 4. Experimental and theoretical internal rotation potential energy functions for propylene sulfide (CPS-CH3). The experimentally observed Raman transitions20 are also shown in the figure. Figure 5. Experimental and theoretical internal rotation potential energy functions for cyclopropylsilane (CP-SiH3). The experimentally observed infrared transitions21 are also shown in the figure.

Figure 6. Experimental and theoretical internal rotation potential energy functions for cyclopropylgermane (CP-GeH3). The experimentally observed infrared transitions22 are also shown in the figure.

Figure 7. Theoretical and refined internal rotation potential energy functions for cyclopropanol (CP-OH). The experimentally observed infrared transitions23,25 are also shown in the figure. The frequencies in parentheses correspond to calculated values.

Figure 8. Theoretical internal rotation potential energy functions for cyclopropanethiol (CP-SH). The calculated values from the CCSD/cc-pVTZ computation are shown. The vibrational frequencies shown are from using the potential energy function calculated from the CCSD/cc-pVTZ computation and the output of the VNCOSPX program.

Figure 9. Experimental and theoretical internal rotation potential energy functions for cyclopropylamine(CP-NH2). The experimentally observed infrared transitions26 are also shown in the figure.

Figure 10. Calculated wavefunctions for CP-NH2 for the experimental energy function from Ref 26.



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Table of Contents Image


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Internal Rotation of Methylcyclopropane and Related Molecules

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