J. Phys. Chem. 1993,97, 6979-6982
6979
Microwave Determination of the Structure of the CsConformation of Dipropyl Ether? Kimberley J. Grant,$ A. R. Hight Walker, and Stewart E. Novick' Department of Chemistry, Wesleyan University, Middletown, Connecticut 06459
Robert K. Bohn, Lou Qi, and Timothy Wheeler Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269-3060
James M. LoBue Department of Chemistry, Ursinus College, Collegeville, Pennsylvania 19426
Mohammad A. Al-Laham AT& T Bell Laboratories, Holmdel, New Jersey 07733 Received: March 10, 1993; In Final Form: April 21, 1993
Microwave spectroscopy of dipropyl ether has been performed by supersonic jet molecular beam electric resonance spectroscopy and by molecular beam pulsed-jet Fourier transform spectroscopy. Calculations suggest that there are four distinct conformations of dipropyl ether that have energies within 0.5 kcal/mol of each other and have geometries of CZU, CZ,C,, and C1 symmetry. The geometry of the C, conformation has been determined. The rotational constants of this conformation are A = 4793.6393(5) MHz, B = 1242.5098(3) MHz, C = 1053.3520(2) MHz, b j = 0.117(6) kHz, b~ = 1.25(5) kHz, AJK= -2.807(7) kHz, AJ 0.448(2) kHz, and AK = 11.57(3) kHz. The outer torsion angles (the O-C-C-C angles) a r e a p p r o ~ i m a t e l y 6 5 ~Unassigned . transitions in the microwave spectrum suggest that more than one additional conformation is present in both C W and pulsed molecular jets.
Introduction Dipropyl ether is a multiconformational molecule. A 1977 gas-phase electron diffraction study suggests that perhaps three conformations are present at room temperature.' These are the all-trans conformer aaaa, where a stands for a dihedral angle in the anti or trans configuration, a conformation of C, symmetry; a g*aag* conformer, where g stands for a dihedral angle in the gauche configuration, a conformation of C2 symmetry; and an aaag conformer of symmetry CI.In addition, molecular mechanics calculations predict that the conformer of C, symmetry (g*aagi) is also low in energy. Indeed, all four of these configurations are predicted to be within 0.5 kcal/mol of each other. Other conformations are also possible. There has been no microwave structuraldeterminationof dipropyl ether attempted before now, probably due to the complication of the simultaneous population of so many conformations. A microwave determination of diethyl ether has been carried out, and the structure of the aa all-trans conformer was solved.2 The authors state that they find many weak transitions which may belong to the other conformers of diethyl ether but that the spectra were so complicated that the analysis was abandoned. We began this study in the hope that the extreme cooling of the supersonicjet molecular beam source would cool the molecule into the lowest energy conformation. This, of course, would greatly simplify the spectrum. The other possibility is that the energy barrier for transformation between the conformations is high enough that the conformational population of the jet is frozen at the room temperature distribution. Each conformation is then separately cooled by the expansion into a low rotational state distribution. Each conformation present at room temperature is then present in the jet, each essentially a separate molecule 7 This paper is dedicated to the memory of E. Bright Wilson, who inspired us all. t Present address: Department of Chemistry, College of Saint Elizabeth, Morristown,NJ 07960.
0022-3654 f 93 f 2097-6979W4.00 f 0
nonconvertableinto the other and each at a rotational temperature near 0 K. This, of course, would also be a simplification over a straight room temperature spectrum and is, in fact, what we observed. This paper presents the determination of the structure of the C, conformation. Fifty-four transitionsranging in rotational quantum number from 2 to 11 have been assigned to this conformation. In addition, we have observed, to date, 35 transitions which we have not yet been able to assign, and which we believe are not due to van der Waals complexes. These are most likely due to one or more of the other conformations. Dipropyl ether is, for us, the first in a series of multiconformational molecules whose structures we will determine by microwave spectroscopy. It was deemed to be simpler spectroscopically than a structurally related molecule which we hope to study in the future: N-nitroso-di-n-propylamine,an environmental pollutant and carcinogen.
Experimental Section The microwave spectrum of dipropyl ether was obtained on two separate instruments. The data were measured using both a molecular beam electric resonance (MBER) spectrometer and a Fourier transform (IT) spectrometer. The A and B stateselecting and state-analyzing fields and the mass spectrometer detector of the MBER apparatus have been described el~ewhere.~,~ The molecular beam pulsed-jet Fourier transform (FT)spectrometer of the Flygare-Balle designs in the laboratory of Prof. Robert Kuczkowski of the University of Michigan is also described elsewhere.6 The two instruments employ different nozzles and thus have different resulting jet conditions. In the MBER apparatus, argon is bubbled through liquid dipropyl ether, C H ~ C H ~ C H ~ O C H ~ C H ~ and C H Sexpanded , through a 25-pm-diameter nozzle with a backing pressure between 0.7 and 1 atm. The vapor pressure of dipropyl ether at room temperature is approximately 60 Torr; thus, the saturated argon vapor contains between 8% and 11% dipropyl ether. The nozzle 0 1993 American Chemical Society
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Grant et al.
The Journal of Physical Chemistry, Vol. 97, No. 27, I993
TABLE I: Dipropyl Ether Frequencies and Assignments for the C. (g+aag) Structure. dev/ dev/ kHz transition freq/MHz kHz transition freq/MHz 12 382.734# 2 3 821.883 10 12 971.126* -1 1 5 263.727 5 13 470.371 5 7 953.695# -3 8 265.1 14’ 13 530.504# -3 0 8 521.146# 0 13 634.443# -2 13 766.877# -2 9 329.685# 1 13 791.558# -2 9 329.844# -2 9 14 454.910 -1 9 550.648 2 15 585.108 9 804.059 0 2 16 537.860 9 968.669# -3 12 16 598.975 10 005.631# 0 10 115.230 -5 17 206.317 5 10 374.308 3 17 287.568 7 17 379.093 10411.024 2 17 10 548.895# 3 17 627.524 -2 10 653.278# 1 221-211 18 044.010 6 53z-523 -8 220-211 10 660.626# 18 142.896 2 432422 0 60s-515 18 144.137 -9 431422 11 097.197# -1 312-202 18 252.492 11 103.375# 3 43d23 -3 221-212 11 220.724# 18 253.740 0 431423 -3 2 ~ 2 1 2 18 506.024 11 228.080# 0 734-726 32r313 18 587.252 1 909-818 11 509.003# -2 -1 321-313 23 292.515 11 545.710# -5 625-514 1 423414 23 322.746 -3 llo,l~-lO~,lo 11 895.814# -3 414-303 23 695.521 11 901.374# 0 144,11-143,11 422-414 23 791.742 12005.406# -2 -7 624-514 24951.492 0 1147-1138 1 918-826 12 111.307 * indicates this line was seen on both the FT and the MBER spectrometer. # indicates this linewasonlyseenon the FT spectrometer. Due to the lower temperature of the jet in the FT apparatus compared to the MBER machine, the lower energy states show up well on the FT, and the higher energy states are observed on the MBER. There are also “beam selection rules” in the MBER experiment that tend to rule out observing some of the low quantum transitions on this spectrometer. @
is just a pin-hole and is continuously open. It is usually assumed that this arrangement produces rotational distributions characterized by a temperature between 5 and 10 K. Spectroscopy is performed while monitoring a mass peak (CH3CH2CH2+) of the dipropyl ether. In the FT apparatus, a 1-mm-diameter nozzle is pulsed. The pulse duration is between 0.25 and 0.75 ms with a repetition rate of 6 Hz. Dipropyl ether was placed in a sample bulb and cooled to 0 OC (vapor pressure 20 Torr), and Ar or a neon-rich mixture of Ne/He was added to obtain a total pressure of 1-1.7 atm. These conditions should produce rotational temperatures of approximately 1 K. Thus, not only should we expect different rotational temperatures in the two experiments; it is also possible that there will be different conformational distributions observed on the two instruments. This will be discussed more fully later. The microwave frequency range available on the FT apparatus is 6-18 GHz; on the MBER, it is 3.16-26.5 GHz. In addition to the zero external electric field microwave transitions on both the MBER and FT machines, Stark effect measurements were performed on the MBER apparatus. A constant electric field (C field) is produced between two parallel, gold-coated Pyrex plates. Microwave radiation is introduced into this C field region through either unterminated X band or tapered G band wave guides oriented so that the microwave electric field vector is parallel to theconstant electric field maintained in the Cfield. Some twisting and spreading of the microwave radiation occur, resulting in the observation of some AM = 1 along with the expected AM = 0 transitions and in some broadening of the transition line widths.
Results Spectroscopic Assignment and Constants. Table I gives the frequencies and spectral assignments of the transitions of the C, conformation of dipropyl ether. Transitions labeled with a #
TABLE 11: Unassigned Transitions’ 3 522.442 15 519.169 3 790.583 15 839.042 16 778.654 4 110.500 18 495.934 7 308.638# (m) 18 723.387 7 345.155# (9) 18 824.638 7 772.596* (m) 18 893.693 8 517.336* (w) 19 117.325 8 828.050 8 920.660* (w) 20 377.492 9203.111# (m) 21 091.309 10429.182 21 100.042 10 522.044* (m) 21 405.588 10 656.576# (s) 21 842.790 10 678.917# (m) 22 004.124 10 808.155# (m) 22 653.637 11 064.834* (w) 24 047.442 12 245.347( (s) 24 148.893 12 540.145# (s) indicates this line was seen on both the FT machine and the MBER. # indicates this line was only seen on the FT machine. (s) intense, (m) medium strength, and (w) very weak on the FT apparatus. MBERobserved intensities are dependent upon A and B field focusing voltages and not simply on population; thus, they are not very useful in intuiting assignments and are left out of this table. @
TABLE 111: Spectroscopic Constants for the C, (g+aaa) Structure‘ A = 4793.6393(5) MHz AJ 0.448(2) kHz B = 1242.5098(3) MHz AK 11.57(3) kHz C = 1053.3520(2)MHz Pa = 0 6 j = 0.117(6) kHz fib 0.79(4) D 6~ 1.25(5) kHz = 0.58(4) D AJK -2.807(7) kHz jtmt = 0.98(4) D a All numbers in parentheses are 1 standard deviation from leastsquares fitting. were measured on the FT apparatus, those with an * were measured on both machines, and the remainder were measured on the MBER apparatus. Table I1 presents other transitions that we have found that are not yet assigned. Some of these transitions will eventually be assigned to one or more additional conformations of dipropyl ether. The transitions in Table I were least-squares fit with a Watson A-reduction asymmetric top Hamiltonian7+*using a program written by Makiag Table I11 gives the spectroscopic constants of the C, conformer. Therotational constants A, B, and C i n Table I11 are consistent with a conformation of C, symmetry. Indeed, the assignments were found by predicting the general values of these rotational constants based on assuming this structure (and others) as generated by molecular mechanics calculations. It was found that the three rotation constants predicted by molecular mechanics for the C, conformation of dipropyl ether were all within 40 MHz of the experimental values. This was an aid in spectral assignment, but due to the presence of transitions from multiple conformations (among which, of course, the mass spectrometer of the MBER was unable to distinguish), this level of prediction was only sufficient for a preliminary assignment. The dipole moment components Pb and cl0 were determined by measuring the frequency splittings and shifts of the 524414,625616,432-423, and 431-423 transitions in the presence of a constant external electric field up to 1000 V/cm. ccP was set equal to zero and not allowed to vary in the fit. A pure second-order Stark effect was assumed.lO As shown in Table 111, Pb and lC were determined to be0.79(4) and 0.58(4) D, respectively, which results in a total dipole moment of 0.98(4) D. Ab Zoitio Calculations. Geometry optimization of the C, conformer of dipropyl ether was performed a t the HartreeFock double-(plus polarization (HF/6-31G**) level of theory.” This level of theory is known to produce reliable results for the geometry optimization.]’ In addition, geometry optimizations at higher levels of theory are not practical at this time due to the size of
Microwave Spectroscopy of Dipropyl Ether
The Journal of Physical Chemistry, Vol. 97, No. 27, 1993 6981
TABLE I V Structural Parameters Relating to the C, Conformation of C H J C H ~ C H ~ ~ C H ~ C H ~ C H ~ bond IengthslA bond angles/deg dihedralsa/deg method C-H 0-C H~C-CHZ HzC-CH~ C-0-C 0-c-C c-c-c C-0c-c 0-c-c-c diethyl ether (pwave)b 1.09 1.408 1.516 112.25 108.37 electron diffractionc 1.116 1.404 1.524 1.524 116.1 109.2 112.1 180 71.8 molecular mechanicsd 1.09-1.12 1.41-1.42 1.53-1.54 1.53-1.54 112-113 108.5-110.0 111.5-112.5 179.8-180.2 62.5-63.6 HF/6-31G** 1.09c 1.397 1.519 1.527 114.74 109.29 113.40 179.75 63.03 this stud# 1.09c 1.36-1.40 1.51-1.54 1.527 115.0-116.7 108.7-108.8 111.8-113.6 180 62.4-64.6 a Zero degrees for the dihedral angles is an "eclipsed" orientation where the two end atoms are on the same side of the bond formed between the two central atoms. Reference 2. The microwave study is of all-trans-diethyl ether. Thus, some of the bond lengths, bond angles, and both of the dihedral angles are not applicable to this row of the table. Reference 1. Both standard MM2 and the commercial program Hyperchem, using various force fields, were employed. The two stated values give the range of values obtained. Average value for all C-H bonds. /These numbers represent the range of values generated in Table V.
TABLE V
Structural Fits of the C, Conformation of CHJCH~CH~OCHZCHZCHJ~ r(O-C)/A ~(H~C-CHZ)/A r(H&-CHz)/A LC-0-C LO-C-C LC-C-C LC-0-C-C LO-C-C-C pb/Da k/Da (1.5 19) 115.0' 108.8' (1.397) (1.527) (113.4') (180') 64.2' 0.801 0.544 (1S27) (1.5 19) 116.0' (1.397) (109.3') 112.8' (180') 64.0' 0.791 0.538 (1 14.74') 108.7' (1.527) (1.5 19) 113.6' (180') (1.397) 64.3' 0.804 0.545 (1.519) 116.1' (109.3') 1.387 (1327) (113.4') (180') 64.6' 0.787 0.537 (1.519) (114.74') 108.7' (113.4') (180') 64.1' 1.399 (1.527) 0.805 0.545 ( 1.519) (114.8') (109.3') b (180') b b (1.527) b b 1.508 116.7' (109.3') (113.4') (180') 64.6' 0.779 0.531 (1.397) (1.527) (114.74') 108.7' (113.4') (180') 64.1' (1.527) 1.521 0.805 0.546 (1.397) 1.537 (114.74') (109.3') 111.8' (180') 63.1' 0.811 0.549 (1.397) (1.527) (114.74') (109.3') (180') (1.527) 1.542 (113.4') 64.5' 0.805 1.365 0.550 (109.3') (180') (1.519) (114.74') (113.4') 62.4' 0.820 0.538 (1.397) (1.527) (114.74') (109.3') (113.4') (63.0') (1.527) (1.519) (180') 0.786 0.539 (1.397) 0 These dipole moments are estimated by taking the bond moments from the ab initio calculation, scaled down 0.85 (see text). These bond moments add to give the pb and pc for the various geometry choices in the table. This combination of parameters to be set and fit produced multiple solutions of dubious numerical significance. Each row of this table represents a separate fit to the three rotational constants. The values in parentheses in each row are not varied in the fit but are set at the calculated ab initio value. The next to the last row in the table is a least-squares fit to the three rotational constants, varying only the dihedral angle. The last row in the table is the ab initio fit. the molecular system. The HF/6-31G** calculations predicted A, B, and C rotational constants that are within 11, 2, and 2 MHz, respectively, of the experimental results. Ab initio methods were also used to calculate the dipole moment of the molecule. The calculated dipole moment of 1.14 D at the HF/6-31G** level is about 16% higher than the experimental value of 0.98(4) D, as given in Table 111. However, the calculated dipole moment at the second-order Moller-Plesset perturbation theory, using the 6-31G** basis set and the HF/6-31G** optimized geometry, is 0.95 D. This is in excellent agreement with the experimental value. The total dipole moment of the molecule was used to obtain the appropriate atomic charges and bond moments at the optimized HF/6-31G** geometry. These bond moments were then scaled down by a factor of 0.85 to account for the above-mentioned overestimation of the dipole moment a t the HF/6-31G** level of theory. In this study, these a b initio results were used to augment our experimental results. The following section explains how we combined these results to arrive at the structure of the C, conformation of dipropyl ether. Structure. Considering only the heavy atoms, and imposing the plane of symmetry which bisects the C-0-C angle, there are eight structural parameters to be determined in the C, conformer of dipropyl ether: three bond angles (C-O-C, 0-C-C, and C-CC), three bond lengths (0-C, H2C-CH2, anf HIC-CH~), and two dihedral angles C-0-C-C (rotation about the 0-C bond, the "inner" dihedral angle) and 0-C-C-C (rotation about the H2C-CH2 bond, the "outer" dihedral angle). Since we have measured only the major isotopomer, we have only three parameters, A, B, and C, that we can use to constrain the geometric fit. There is also structural information implicit in the two measured dipole moment components, @b and w,. Table IV presents structural parameters of diethyl ether from a microwave study' and of dipropyl ether from a gas-phase electron diffraction study: from an MM2 molecular mechanics calculation, and from the ab initio calculation. As mentioned above, this calculation
resulted in relatively accurate rotational constants. Thus we felt justified in using some of the structural parameters from this calculation to augment the experimental rotational constants in aid of finding a structural fit. We proceeded to find a representative structural fit to the rotation constants by fixing all but three of the structural constants at the a b initio values and fitting the remaining three bond lengths/angles in a least-squares sense to A, B, and C. The inner dihedral angle, rotation about the O-C bond, always came to within l o of 180O. We chose, therefore to set this angle to 180° for all fits. (180° for a dihedral angle is the trans or anticonfiguration,where the two outer atoms of the four atoms that define the dihedral angle are a t their maximum distance from each other). Also set was the H,CCHI bond distance, which did not vary significantly in the fits. All C-H distances were set to their ab initio values. Thus, with the imposition of C, symmetry, there are six molecular parameters to be fit. Since we are concerned with the conformations of dipropyl ether, we will always choose to fit the outer dihedral angle (rotation about the H~C-CHZbond). We can now fit any two of the remaining five geometrical parameters. There are 10 ways to choose which two to fit and which three to set (at their ab initio values). All 10 independent fits are presented in Table V, with each row in the table being a separate fit. The values in parentheses are the ab initio values which are not varied in that particular fit. The 11th row in Table V is a least-squares fit to the three rotational constants varying only the outer dihedral angle. The 12th row in the table is the ab initio structure. Also presented in Table V are the two dipole moment components that are estimated by taking the bond moments deduced from the ab initio calculation. The dipole components of each of the 11 fits give pc within experimental error; the pb)s are a bit on the high side, but all are within 2 standard deviations of the experimental Pbvalue. Thus, our hope of being able to use the dipole components to distinguish between the various fits has not been realized. This is basically because all the geometric fits presented in Table V are similar to each other. The microwave data are actually being
6982 The Journal of Physical Chemistry, Vol. 97, No. 27, 1993
Figure 1. Perspective drawing of the C, conformation of dipropyl ether. The purpose of the drawing is to clearly define the 0-C-C-C dihedral angle, labeled x. used to confirm and refine the results from the HF/6-31G** calculation of the C,conformation of dipropyl ether. The range of values generated by these various fits is presented in the last row in Table IV. Figure 1 presents a perspective drawing of the C, conformation of dipropyl ether. Some of the bonds in this drawing are foreshortened due to the perspective; the major purpose of the drawing is to clearly define the 0 4 4 - C dihedral angle, labeled x in the figure. Unassigned Transitions and the Barrier to Conformational Conversions. There are enough transitions that are unassigned to account for the expected three additional conformations: Cb, C2,and C1. (The reason, of course, that there are so many more assigned (C,) transitions than unassigned (Cb,C2,and Cl)is that once high-resolution spectroscopicconstants are known additional weaker transitions are easy to find.) The relative intensities of the transitions (both assigned and unassigned) differ on the FT and the MBER apparatus. This is in part due to the relative populations within a conformation, as we recognize when we recall that the beam temperature in the pulsed valve of the FT spectrometer is of the order of 1 K or less and that of the CW jet in the MBER apparatus is between 5 and 10 K. However, there could also be a contribution due in part to the relative populations of the conformations in the two instruments. This is not a temperature effect, per se, since at even 10 K the relative population of a conformation of 0.5 kcal/mol above the lowest
Grant et al. energy conformation would only be 10-l’ and absolutely undetectable. However, at room temperature, the relative population of this conformation would be 0.4 and quite easy to detect. So the question is, “what is the effective conformational temperature in the two experiments?”. The two extreme cases are (1) the conformational temperature cools in the jet to the final rotational temperature and (2) the conformational populations are frozen into the room temperature distribution that they had before the expansion. It seems likely that the expansion in the pulsed nozzle of the FT apparatus has a better chance of affecting conformational cooling than does the CW nozzle of the MBER instrument. This is because of the relative diameters of the two nozzles: 25 pm for the CW nozzle and 1000 pm for the pulsed nozzle. Both jets use approximately the same backing pressure on the order of 1 atm of Ar (and sometimes Ne). The molecules cool with increasing Mach number until the Mach shock disk is reached.’* All things being equal, the distance from the nozzle to the Mach disk is proportional to the nozzle diameter. Thus, the molecules in the pulsed jet undergo cooling collisions over a distance that is a factor of 40 greater than the distance traveled by the molecules in the CW jet. In a case like ours where the conformational energy differences are on the order of kT for room temperature, the initial collisions (the first five nozzle diameters downstream?) can affect the conformational populations. Thus, we expect that the highest energy conformation that is populated in the CW jet might have a negligible population in the pulsed jet.13 Chemical intuition and molecular mechanics calculations suggest that the lowest energy conformation will be the all-trans Cb structure. This structure will have a sparse microwave spectrum, and it is not unreasonable that transitions from this conformation (if found at all) remain in the “unassigned”category. The C,conformation shows up well in both experiments, with the MBER (CW jet) instrument tending to find higher J transitions and the FT (pulsed jet) apparatus tending to find lower J transitions. We can speculate that when all the conformations are assigned the highest energy conformation appearing in the MBER apparatus will be very weak or absent in the FT spectrometer.
Acknowledgment. This work was sponsored by the Yankee Ingenuity Initiative, Apollos Kinsley Collaborative Grants, Connecticut Innovations, Inc., Department of Economic Development, Grants 91K006 and 92K004. We thank Prof. Robert Kuczkowski of the University of Michigan, who graciously allowed one of us (R.K.B.) the use of his Fourier transform microwave spectrometer. References and Notes (1) Astrup, E. E. Acta Chem. S c a d . A 1977, 31,4. (2) Hayashi, M.; Kuwada, K. Bull. Chem. Soc. Jpn. 1974, 47, 3006. (3) LoBue, J. M.; Rice, J. K.; Novick, S. E. Chem. Phys. Lett. 1984, I 12, 376. (4) LoBue, J. M.; Rice, J. K.; Blake, T. A.; Novick, S. E. J. Chem. Phys. 1986, 85,426 1. (5) Balle, T. J.; Flygare, W. H. Rev. Sci. Instrum. 1981, 52, 33. (6) Hillig, K. W. 11.; Matos, J.; Scioly, A.; Kuczkowski, R. L. Chem. Phys. Lett. 1987, 133, 359. (7) Watson, J. K. G. J. Chem. Phys. 1967, 46, 1935. (8) Watson, J. K. G. Vibrational Spectra and Structures 1977, 6, 1. (9) Maki, A. G.,private communication. (10) Beaudet, R. A. computer program, private communication. (1 1) For a general introductionto the Hartree-Fock-based methods, see:
Hehre, W. J.; Radom, L.; Schleyer,P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (12) Miller, D. R. In Atomic and Molecular Beam Methods; Scoles, G., Ed.; Oxford University Press, 1988; Vol. 1, p 15. (13) Ruoff, R. S.; Klots, T. D.; Emilsson, T.; Gutowsky, H. S. J. Chem.
Phys. 1990, 93, 3142.
