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Molecular Structure and Conformations of 1,2-Dimethoxycyclobutene-3,4dione. An Electron-Diffraction Investigation Augmented by Quantum Mechanical and Normal Coordinate Calculations Luke L. Costello, Lise Hedberg, and Kenneth Hedberg J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp507648j • Publication Date (Web): 26 Aug 2014 Downloaded from http://pubs.acs.org on August 31, 2014

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The Journal of Physical Chemistry

Molecular Structure and Conformations of 1,2-Dimethoxycyclobutene-3,4dione. An Electron-Diffraction Investigation Augmented by Quantum Mechanical and Normal Coordinate Calculations Luke L. Costello, Lise Hedberg, and Kenneth Hedberg *

Department of Chemistry, Oregon State University, Corvallis, Oregon 97331-4003

Received:

Keywords: gas phase interatomic distances isomeric composition vibrational amplitudes DFT calculations

*

Corresponding author. email: [email protected]; tel: 541-737-6734; FAX: 541-737-2062

ACS Paragon Plus Environment


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-2Abstract The structure and conformations of 1,2-dimethoxycyclobutene-3,4-dione in the vapor at a temperature of 185 °C have been measured by gas-phase electron-diffraction. The molecule exists in two forms, one of symmetry C2v with the methyl groups trans to the double bond, and one of Cs symmetry with a methyl group cis and the other trans to this bond (these forms hereafter designated as trans and cis). The molar ratio trans/cis is 68/32 with a 2F uncertainty of about 24. Many of the parameter values for the two forms are very nearly alike and could not be measured experimentally. With the adoption of parameter differences calculated at the B3LYP/cc-pVTZ level, the following bond distances (rg/Å) and bond angles (p/deg) with estimated 2F uncertainties were obtained for trans/cis. C1=C2 = 1.381(9)/1.381, C1–C4 = 1.493(11)/1.495, C3–C4 = 1.543(20)/1.545, C=O = 1.203(4)/+1.200,, C1–O = 1.316(6)/+1.320,, O–CH3 = 1.444(9)/+1.443,, C=C–C3 = 93.1(5)/+93.1,, C3–C4=O = 136.7(29)/+136.9,, C=C–O = 131.0(23)/137.5 and 131.8, C–O–C = 117.2(12)/ 118.2 and 116.9; the individual angle values for the cis form listed as averages differ very little. The bond distances and bond angles are in excellent qualitative agreement with prediction based on conventional ideas about the effects of conjugation and hybridization, and their relative values agree very well with predictions from quantum mechanical calculations.


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-3Introduction

The effect of substituents on the size and shape of the cyclobutene (CB) ring has been of interest to us for many years. This interest developed when the rg length of the C3-C4 bond in hexafluoroCB was found to be 1.596 (16) Å from a gas-phase electron-diffraction (GED) investigation1, much greater than the expected 1.54 Å - 1.56 Å. In a GED reinvestigation2,3 we found the bond length at 1.584 (11) Å to be a bit less than the earlier value, but still longer than in perfluorocyclobutane (1.567 (8) Å)1 as well as that in cyclobutane itself (1.554 (1) Å).4 Subsequent GED investigations of other molecules with cyclobutene (CB) skeletons led to values for the C3-C4 bond equal to 1.599 (10) Å, 1.567 (12) Å, and 1.581 (21) Å in, respectively, 1,2-dichloro-3,3,4,4tetrafluoroCB,5 1,2-dimethoxy-3,3,4,4-tetrafluoroCB,6 and 1-methoxypentafluoroCB.7 Despite the large uncertainty associated with the monomethoxy compound, it appears that the long C3-C4 bond occurs only with halogen substitution at the 1,2 positions; for example, its value is only 1.539 (6) Å in 3,3,4,4-tetrafluoroCB.8 All the structures described above contained an F2C-CF2 group opposite the double bond. An evident question is what the effect on the C3-C4 bond might be if the fluorines were replaced with other substituents. The work described here on the molecule 1,2-dimethoxyCB-3,4-dione (dimethoxyCBdione, Figure 1) provides evidence of the effect of replacing the pairs of fluorines with oxygens. Other studies on similar molecules are planned.

Experimental

The diffraction experiments were done with the Oregon State apparatus at a nozzle-tip temperature of 185 °C. The sample from TCI (designated 98+ % pure) was used as received.



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-4Conditions of the experiments were as follows. Film:

Kodak

Electron

Image,

8"x10";

development: 10 min in D19 developer diluted 1:1; exposures, 60-90 s at the long camera (LC) distance and 60-120 s at the middle distance (MC); beam current: 0.8 :A; nominal accelerating Figure 1.

potential, 60 kV calibrated against CO2 (ra(CO) =

1.1646 Å and ra(OO) = 2.3244 Å); sector function, angular opening % r3. Four films from the LC and two from the MC were traced, the resulting digitized data reduced, and averages from each camera distance formed. Details of the procedures used to obtain these curves may be found in previous publications from this laboratory. Calculations concerning the data reduction and generation of intensity and radial distribution curves have been described.9 ,10.

Theoretical calculations

As Fig. 1 suggests, the molecule dimethoxyCBdione is formally a double rotor involving torsion around the bonds from the methoxy groups to the ring. It is evident, however, that such rotation would be strongly restricted by steric interaction between the two methyl groups. Molecular orbital calculations that were carried out with use of several basis sets at various levels of theory revealed minimal energy conformations comprising an trans-trans and a cis-trans isomer (trans and cis refer to the orientation of the H3C– bond with respect to the C=C bond); these forms will hereafter be called trans and cis. We have taken the results from the B3LYP/cc-pVTZ calculations as the source of certain auxiliary information used as constraints in the models for the trans and the


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-5cis isomers of the dimethoxyCBdione molecule. Normal coordinate calculations were also necessary in order to generate other quantities intended for use in the structural analysis: the perpendicular amplitudes, centrifugal distortions, and certain vibrational amplitudes. These calculations were done with use of the program ASYM40.11,12

Structure Analysis

Model. Since the heavy-atom structures of the trans and cis forms of the molecule have different symmetries, the parameter values for the bonds and bond angles mapped from one conformer to the other must differ. However, according results from the theoretical calculations, these differences are so small—with the possible exception of the methoxy group angle at the oxygen atom—that there is no hope of evaluating them experimentally.

A system model for

dimethoxyCBdione that includes both forms of the molecule thus requires the introduction of constraints. This model was based on the dominant trans form with the following parameters (the atom numbering is seen in Fig. 1). +r(C–H),, r(C=C), r(C1–C4), r(C3–O7), r(C1–O5), r(O5–C9), p(C1–C2–C3), p(C4–C3=O7, p(C2=C1–O5), p(C1–O5–C9), and +p(O5–C9–H),. The cis form was included by invoking the differences between the values of corresponding parameters for the two forms to define the structure of the cis molecule; the relative amount of the two forms was investigated by a mole fraction parameter. There are also vibrational amplitude parameters which were handled as described below. Refinement of the structure. The refinements were done by our usual least squares method based on fitting theoretical scattering curves to the experimental ones. The model was defined in r" space making use of the calculated correction terms to generate the ra values used in the fitting



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-6process. The amplitude parameters were refined in groups. The members of each group were given values that differed by the amounts predicted from the normal coordinate calculations which made use of the theoretical quadratic force fields determined by B3LYP/cc-pvtZ theory. The final model was defined by the 11 geometrical parameters listed above and by eight vibrational parameters; it was found that the carbon-hydrogen bond amplitude did not refine to a reasonable value and so it was fixed at the value predicted from theory. With the assumption about the carbon-hydrogen bond amplitude in place, the refinement process converged smoothly. Figure 2 shows the final scattered intensity curves, Table 1 the final results for bond angles and distances, and Table 2 the correlation matrix for these parameters. Table S1, found as Supplementary Material, contains all non-bond distances between the heavy atoms and their associated amplitudes of vibration. The amplitude groupings mentioned above are apparent in Tables 1 and S1, and the various distance corrections mentioned above may be deduced by comparing the values of the different distance types.13

Figure 2.


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-7Table 1. Values of bond distances r /Å), amplitudes of vibration (l /Å), and bond angles (p/deg) for the Cs and C2v forms of 1,2-dimethoxy-3,4-cyclobutenedione experimentala

theoreticalb

trans (C2v)c

parameter r"

ra

rg

cis (Cs)d l

r"

ra

rg

l

trans

cis

re

re

bond distances and amplitudes +C-H, C=C

1.089 (17) 1.101 1.376 (9) 1.379

1.107 1.381

0.076 0.049 (4)

1.089 1.101 1.107 1.376 1.38 1.381

0.076 0.05

1.087 1.381

1.087 1.382

C1-C4 C2-C3

1.487 (11) 1.49

1.493

0.058

1.489 1.492 1.495 1.489 1.493 1.495

0.059 0.059

1.489

1.490 1.490

C3-C4

1.535 (20) 1.541

1.543

0.063

1.537 1.542 1.545

0.063

1.550

1.552

C3-O7 C4-O8

1.187 (4)

1.201

1.203

0.042 (4)

1.186 1.202 1.204 1.182 1.196 1.197

0.042 0.042

1.202

1.202 1.197

C1-O5 C2-O6

1.307 (6)

1.314

1.316

0.048

1.306 1.312 1.314 1.315 1.323 1.325

0.048 0.048

1.315

1.313 1.323

O5-C9 O6-C10

1.407 (9)

1.442

1.444

0.057

1.402 1.436 1.439 1.407 1.434 1.446

0.056 0.057

1.447

1.442 1.447

[

O

bond angles pC1-C2-C3 pC2-C1-C4

93.1 (5)

93.0 (5) 93.2

93.2

93.2 93.3

pC2-C3-C4 pC1-C4-C3

86.9 (5)

87.0 (5) 86.8

86.8

86.8 86.7

pC4-C3-O7 pC3-C4-O8

136.7 (29)

137.0 (29) 136.8

137.4

137.8 137.5

pC2-C3-O7 pC1-C4-O8

136.4 (29)

136.0 (29) 136.4

135.8

135.5 135.8

pC2-C1-O5 pC1-C2-O6

131.0 (23)

137.5 (23) 131.8

130.9

137.4 131.7

pC1-O5-C9 pC2-O6-C10

117.1 (12)

118.2 (12) 116.9

116.6

117.7 116.4

+pO-C-H,

109.9 (30)

109.9 (30)

108.7

108.7

X R

f

0.68 (24) ———————————

0.097

0.32 (24) ——————————



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-8Footnotes to Table 1 a

Quantities in parentheses are 2F uncertainties and include estimates of the effects of correlation among the data.

Quantities in square brackets were refined as groups. The italicized C–H amplitude was assumed. bB3LYP/cc-pVTZ. c

Uncertainties for rg and ra assumed the same as for r" .

d

e

Assumed. fMole fraction. gQuality-of-agreement factor: R = [3iwi)i2/3iwi(siIm,i(obsd))2]½ where )i = siIm,i(obsd) -

Uncertainties assumed equal to those of the C2v form.

siIm,i(calc.).

Table 2. Correlation Matrix x 100 for Structure-Defining Parameters of 1,2-Dimethoxycyclobutene-1,2-dione

FLSa

1

1 +r(C-H),

0.006

100

2 r(C=C)

0.003

3 +r(C-C),

0.004

-16

4 +r(C-O),

0.002

67

-29

-33 100

5 +r(C=O),

0.001

48

-35

-32

47 100

6 r(O-CH3)

0.003

38

-58

-80

61

52 100

7 +pC=C-C,

0.176

6

-73

-85

20

17

8 +p(C=C-O),

0.818

13

21

20

35

-35

-16

-15 100

9 +p(C-C=O),

1.021

7

21

27

7

17

-13

-17

11 100

10 +p(C-O-C,

0.410

-4

24

30

-12

-24

-42

-27

32

11 p(O-CH3)

1.057

-1

-8

-1

28

-10

-1

-1

42

a

2

3

4

5

6

7

8

9

10

11

-4 100 52 100

70 100

-19 100 -7

39 100

Standard deviation from least squares. Distances (r) in angströms; angles (p) in degrees.


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-9-

Figure 3

Discussion

The matters of greatest interest in our investigations of the substituted cyclobutenes are the bond lengths in the four-member rings. Comparison of these and the bond lengths to heavy atom ligands in the several molecules we have studied provides a consistent view of the nature of the



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- 10 bonding. Table 3 presents a summary of the data which may be interpreted as follows. The C3–C4 bond in dimethoxyCBdione lies between two carbonyl groups and is thus an sp2–sp2 conjugated

Table 3. Bond-length Values/Å in Substituted Cyclobutenes theory (re)a

experiment (rg) molecule

C3–C4

C1=C2

hexafluoroCBb

1.585(8)

dichlorotetrafluoroCBc

1.599(10) 1.359(9)

dimethoxytetrafluoroCBd

C1–C4

C1–O5

1.570

1.334

1.515

1.572

1.341

1.508

1.570(11) 1.338(21) 1.499(8) 1.329(12)

1.559

1.355

1.494

1.332

dimethoxyCBdionee

1.544(20) 1.381(9)

1.552

1.371

1.490

1.316

cyclobutenef

1.566(3)

a

1.328(24) 1.503(5)

C3–C4 C1=C2 C1–C4 C1–O5

1.500(6)

1.494(11) 1.317(6)

1.342(4)

B3LYP/cc-pVTZ. bRef 2. cRef 5. dRef 6. eThis work; the values are weighted averages.

f14

single bond which is known to be shorter than sp3–sp3 bonds, the type that comprises C3–C4 in the other molecules. On the other hand, the C1=C2 bond in dimethoxyCBdione is significantly longer (0.05 Å) than in the other molecules which implies weaker bonding, i.e., that it is of a smaller bond order. The reduced electron density here should lead to an increased density (higher bond order) in adjacent bonds. This is indeed the case: the C1–O and C2–O bonds are much shorter (0.10 Å) than the rough estimate of 1.42 Å for the length of a C–O single bond.15 The C1–C4 and C2–C3 bonds should also be shorter in dimethoxyCBdione because they are formally sp2–sp2 conjugated single bonds. However, the length of bond C1=C2, which is much greater than that of a conventional double bond, implies a hybrid character in the direction of sp3 for the bonds around these atoms. In the limit the C1–C4 and C2–C3 bonds would be of type sp2-sp3 which one expects would have a


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- 11 length intermediate between 1.57 Å (cyclobutene16) and 1.47 Å (acrolein17). At 1.49 Å in dimethoxyCBdione, this is indeed the case. It is satisfying that the classical ideas about the nature of the bonding in these molecules accounts so nicely for the experimentally measured lengths of the bonds themselves. It is also pleasing that the experimental results are in qualitative agreement with high quality molecular orbital theory.18 Thus, from B3LYP/cc-pVTZ optimizations the C1=C2 bond in dimethoxyCBdione is about 0.03 Å longer than in the two perhalo compounds and the C1–O and C2–O correspondingly shorter than a single bond. Also, the C3–C4 bond is shorter than in the other molecules as are the C1–C4 and C2–C3 bonds. Lastly, C1–C4 and C2–C3 are slightly shorter in dimethoxyCBdione than in dimethoxytetrafluoroCB as would be expected from the absence of conjugation in the latter. The structural interpretations discussed above obviously depend upon the reliability of the measured parameter values which deserves comment. Perhaps the most important item affecting these values is the set of assumptions invoked to broaden the GED analyses, i.e., to enable estimates of certain parameter values that would be inaccessible without them. For example, values for the distance types r" and rg are produced from the experimental ra by a normal coordinate calculation that makes use of a quadratic force field derived from a quantum mechanical frequency calculation; thus the corrections ignore anharmonicity and imply that the level of quantum mechanical theory is appropriate. Further, some amplitudes of vibration that cannot be refined are assumed to have the theoretically calculated values. These have the effect of making the estimates of parameter-value uncertainty smaller than they otherwise would have been. Despite these caveats, our interpretations of the results for the molecules listed in Table 3 are consistent across the group and suggest that the ideas expressed are sound.



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- 12 .Conclusions

The heavy atom framework of the molecule 1,2-dimethoxycyclobutene-3,4-dione is planar in the gas, and the bond lengths are consistent with the existence of conjugation and/or sp2 type bonding between the carbonyl groups, and partial double-bond character for the links between the carbon and methoxy oxygens with consequent reduced bond order and lengthening of the carbon carbon double bond.

Acknowledgment. This work was supported by the National Science Foundation under grant CHE 0613298.

Supplementary Information Available: A table of the experimental scattered intensities as used in the refinements and and their mathematical form as used in the structure refinements, and a table containing the experimental values of nonbond distances (r", ra, rg) and rms amplitudes of vibration (l); and the corresponding theoretical values of distances (re). This informtion is available free of charge via the internet at http://pubs.acs.org


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- 13 References and Notes

(1) Chang, C. H.; Porter, R. F.; Bauer, S. H. The Molecular Structures of Perfluorocyclobutane and Perfluorocyclobutene, Determined by Electron Diffraction. J. Mol. Struct. 1971, 7, 88-99. The distance given is our estimate of rg, about 0.001Å larger than the that reported by the authors, which is apparently ra. (2) Császár, A; Hedberg, K. Hexafluorocyclobutene: Gas-Phase Molecular Structure and Quadratic Force Field from an Electron-Diffraction and ab Initio Study. J. Phys. Chem. 1990, 94, 3525-3531. (3) Hedberg, L.; Hedberg, K. Hexafluorocyclobutene: Is the C3– C4 Single Bond Abnormally Long? J. Phys. Chem. 1993, 97, 10349-10351. (4) Egawa, T.; Fukuyama, T.; Yamamoto, S.; Takabayashi, F.; Kambara, H.; Ueda, T.; Kuchitsu, K. Molecular Structure and Puckering Potential Function of Cyclobutane Studied by Gas Electron Diffraction and Infrared Spectroscopy. J. Chem. Phys. 1987, 86, 6018-6026. (5) Thomassen, H.; Hedberg, K. 1,2-Dichlorotetrafluorocyclobutene: Gas-Phase Molecular Structure and Quadratic Force Field from an Electron-Diffraction and ab Initio Study. J. Phys. Chem. 1990, 94, 4847-4850. (6) Richardson, A. D.; Hedberg, K.; Lunelli, B. The Puzzle of Bond Length Variation in Substituted Cyclobutenes. A New Example: Molecular Structure and Conformations of 1,2-Dimethoxy-3,3,4,4-tetrafluorocyclobut-1-ene. J. Phys. Chem. A 2010, 114, 5358-5364. (7) Frogner, M.; Hedberg, K.; Hedberg, L.; Lunelli, B. Molecular Structure and Conformations of 1-Methoxy-2,3,3,4,4-pentafluorocyclobut-1-ene. J. Mol Struct. 2010, 978, 294-298. (8) Andrews, A. M.; Maruca, S. L.; Hillig II, K. W.; Kuczkowski, R. L. Microwave Spectrum and Structure of 3,3,4,4-Tetrafluorocyclobutene. J. Phys. Chem. 1991, 95, 7714-7717. (9) Gundersen, G.; Hedberg, K. The Molecular Structure of Thionyltetrafluoride, SOF4 J. Chem. Phys. 1969, 51, 2500-2507. (10) Hedberg, L. Determination of Molecular Structures by Analysis of ElectronDiffraction Data. Method for Automatic Removal of Background. Abstracts, Fifth Austin Symposium on Gas Phase Molecular Structure, Austin, TX, March 1974, No. T9. (11) Hedberg, L.; Mills, I. M. Harmonic Force Fields from Scaled SCF Calculations: Program ASYM40. J. Mol. Spectrosc. 2000, 203, 82-95.



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- 14 (12) Hedberg, L.; Mills, I. M. ASYM20: A Program for Force Constant and Normal Coordinate Calculations, with a Critical Review of the Theory Involved. J. Mol. Spectrosc. 1993, 160, 117-142. (13) The formulas are rg = r" + [+)x2, + +)y2,]/2r and ra = rg + l2/r where )x and )y are displacements of the nuclear pair perpendicular to an axis joining them and l2 is the mean square amplitude of distance change due to vibration. (14) Bak, B.; Led, J. J.; Nygaard, L.; Rastrup-Andersen, J.; Sørensen, G. O. Microwave Spectra of Isotopic Cyclobutenes. Molecular Structure of Cyclobutene. J. Mol. Struct. 1969, 3, 369-378. (15) Schomaker, V.; Stevenson, D. P. Some Revisions of the Covalent Radii and the Additivity Rule for the Length of Partially Ionic Single Covalent Bonds. J. Am. Chem. Soc. 1941, 63, 37-40. (16) Bak, B.; Nygaaard, L.; Rastrup-Andersen, J.; Sørensen, G. O. Microwave Spectra of Isotopic Cyclobutenes: Molecular Structure of Cyclobutene. J. Mol. Struct. 1969, 3, 369-378. (17) Blom, C. E.; Grassi, G.; Bauder, A. Molecular Structure of S-cis- and S-transacrolein Determined by Microwave Spectroscopy. J. Am. Chem. Soc. 1984, 106, 7427-7431. (18) Quantitative comparisons are of little use since the re values from theory usually differ by a few hundredths of an angström from the experimental rg.


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- 15 Figure captions. Figure 1. Diagram of the trans-trans (C2v) and cis-trans (Cs) forms of 1,3-dimethoxycyclobutene-3,4-dione with atom numbering.

Figure 2. Intensity curves. The difference curves are experimental minus theoretical for the model defined by Table 1.

Figure 3. Radial distribution curves. The vertical lines indicate the distance values in the C2v form; the major changes from trans to cis are shown by the labeled dotted lines. The difference curve is experimental minus theoretical for the final model.



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- 16 Graphic for Table of Contents:


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8

7 4

12 9

3

1

2

5

11

6

10

9’ 11’

12’



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

experimental long camera middle camera

differences long camera

5

10

15

20

-125

s/Å

middle camera

30

35

40


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experimental

0

1

C9O6 C3C9

C9O8 C4C9 C2C9

bond distances geminal distances 2

C9C10 O7C9

3

r/Å

4

5

difference


6

r/Å

Molecular Structure and Conformations of 1,2 ... - ACS Publications

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