The C–D Stretch of Monodeuterated Propargyl Radical (CH2CCD)

JOURNAL OF MOLECULAR SPECTROSCOPY ARTICLE NO.

186, 193–202 (1997)

MS977437

The C–D Stretch of Monodeuterated Propargyl Radical (CH2CCD) Wade C. Eckhoff, Charles E. Miller, 1 Charles F. Billera, Paul S. Engel, and R. F. Curl Chemistry Department and Rice Quantum Institute, Rice University, Houston, Texas 77005 Received August 8, 1997

The high-resolution infrared spectrum of the monodeuterated propargyl radical ( CH 2CCD ) has been obtained in the region of its acetylenic C – D stretch. Lower state rotational constants were determined for the molecule. The upper state was significantly perturbed making the upper state rotational constants determined much more uncertain. q 1997 Academic Press

1. INTRODUCTION

The propargyl radical (H2CCCH) has attracted interest as one of the simplest possible conjugated organic free radicals. Observed in abundance in pyrolysis and oxidation processes (1–9), it is thought to play an important role in combustion and soot formation (10–12). Ramsay and Thistlethwaite first observed an electronic absorption band of propargyl around 30 000 cm01 , but the observed lines were diffuse and definite assignments could not be made due to predissociation in the upper state (13). Later, Dubois and LeClercq (14) observed another diffuse absorption region with a lmax near 250 nm. Recently Fahr et al. have measured the absorption cross-section of Dubois and LeClerq’s system (15). Lossing measured a potential of 8.68 eV for propargyl ionization by electron impact ( 16) and Minsek and Chen obtained a value of 8.67 eV by photoelectron spectroscopy (17). Robinson et al. measured the electron affinity of the propargyl radical through negative ion photoelectron spectroscopy (18). The electron spin resonance measurements of Kasai show that the vibrationally averaged propargyl structure has C2£ symmetry (19), in agreement with theoretical calculations (20–27). Matrix isolation studies have been reported by Jacox and Milligan (28), Mal’tsev (29), and Huang and Graham (30) for the normal species of the propargyl radical as well as for some deuterated species. Despite its possible importance in the combustion process and subsequent soot formation, gas-phase spectroscopic data on propargyl remain scarce. The first rotationally resolved spectrum of the propargyl radical was reported by Morter et al. who investigated the n1 acetylenic CH stretching band of the normal species near 3322 cm01 by the technique of infrared laser kinetic spectroscopy (31) and subsequently studied the propargyl recombination rate (32). Recent stud1

Current address: Chemistry Department, Haverford College, Haverford, Pennsylvania 19041.

ies by Tanaka and co-workers have looked at the CH2 wagging band near 698 cm01 (33, 34). The microwave spectrum has been studied by Tanaka et al. (35). There have been no high-resolution spectroscopic investigations of isotopically substituted propargyl radicals; therefore, we undertook a study of the n1 C–D stretching mode of d1-propargyl (CH2CCD) using the technique of infrared kinetic spectroscopy. 2. EXPERIMENTAL

The experimental arrangement is pictured in Fig. 1. An early prototype of this apparatus has been described elsewhere (36), so only a brief summary will be given here. The 23 W all lines cw output of an argon ion laser (Coherent Innova 200-25) is split and simultaneously pumps two single frequency titanium:sapphire ring lasers (Coherent 899-21 and 899-29). These lasers are actively frequency stabilized by locking to external e´talons, resulting in typical linewidths of 500 kHz. The output of these lasers is appropriately attenuated with neutral density filters to 500 mW from each laser. The polarization of the high frequency laser is rotated by 907 using a polarization rotator and the two beams are spatially overlapped with a polarization cube. The collinearly propagating beams are focused into a 4 1 4 1 45 mm AgGaS2 crystal where type I 907 phase matching takes place and infrared light is generated at the difference between the two input frequencies. The propargyl radical was produced by flash photolysis of d1-propargyl bromide (a description of the synthetic route is provided in Appendix) at 193 nm using a Lambda Physik EMG 101 ArF excimer laser specially modified to allow use of an external recirculator system to increase pulse power reproducibility (37). For the photolysis, a mixture consisting of approximately 8 mTorr of d1-propargyl bromide with about 7.5 Torr of He buffer gas was flowed slowly through the infrared absorption cell. This cell is a 1 m long multiple reflection (White) (38)) cell adjusted to a 36 m total path

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FIG. 1. Experimental arrangement.

length with a UV/IR overlap path length of approximately 10 m. The transmitted infrared light is focused on a liquidnitrogen-cooled InSb detector. To reduce the noise that arises from fluctuations in laser intensity, the output of the infrared probe was balanced against a reference beam, which was split off the main probe beam before it was passed through the absorption cell. The reference beam was focused onto a second detector with reversed polarity with respect to the probe beam detector. The two detectors are connected so that, at the input of a single transimpedance amplifier connected to the junction between the two detectors, the photocurrent generated by the signal detector is canceled by the photocurrent generated by the reference detector. An amplifier output voltage is thus produced only when the currents flowing through the two detectors is unequal. Balancing was achieved by manually adjusting the power of the beam striking the reference detector with a MgF2 polarizer such that the laser noise was minimized. To obtain the absorption signal of the propargyl radical, a transient digitizer acts as a dual-gated integrator, averaging signals from 5 to 15 ms before the excimer flash and subtracting this value from the signal averaged from 25 to 35

ms after the flash. The residual low frequency noise was largely eliminated by this subtraction, thereby roughly tripling the S/N ratio. A transient digitizer sampling rate of 5 MHz was used throughout. The IR spectrum was obtained by fixing the frequency of one of the lasers and scanning the second. The system was limited to 2 cm01 scans because of the phasematching condition. The scans started with poor phase matching producing low infrared power at the beginning of a scan; the IR power reached a maximum with good phasematching near the middle of the scan, and then decreased again as phasematching was lost. The IR power produced had a FWHM of 1 cm01 . By changing the frequency of the fixed laser after each run, sequential scans that completely cover the frequency range of the propargyl n1 absorption are acquired. Within each scan, signals were averaged over 16 excimer shots, the laser stepped by 50 MHz, and the process repeated. Because the scanning laser could only continuously scan 10 GHz under computer control of the Autoscan II system, we simultaneously obtained the spectrum of a 750 MHz e´talon. The e´talon peaks allow 10 GHz segments to be fitted together with an accuracy of 0.001 cm01 and in this fashion we were able to piece together successive 10 GHz segments to generate

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FIG. 2. Sample portion of the spectrum.

continuous 2 cm01 scans. Frequency calibration was accomplished by splitting off a portion of the infrared probe and obtaining the absorption spectrum of N2O simultaneously with the probe signal and external e´talon. On average, this reference gas provided six strong absorption signals over the course of a scan and provided absolute frequency calibration to within 0.001 cm01 . Line positions for N2O were taken from the GEISA database (39). 3. RESULTS

Samples of the n1 acetylenic stretch spectrum are shown in Figs. 2 and 3. The most intense lines represent approximately 0.3% absorption of the infrared probe. Levels of absorption this small are beginning to approach the current sensitivity limits of the apparatus. Previous investigations have shown that propargyl has C2£ symmetry with the twofold axis corresponding to the a-axis. As the dipole transition moment for the C–D stretch of CH2CCD is expected to be along the aaxis, the spectrum was expected to (and did) consist of parallel transitions ( DKa Å 0) of a nearly symmetric prolate rotor. (Henceforth we will drop the subscript and use K for Ka . The ground state of propargyl is 2 B1 , implying that the statistical weights of the even K to odd K levels are expected to be in the ratio 3 to 1. Relative intensity calculations indicate that the strongest series are expected to correspond to K Å 0, 2, and 4. K Å 3 was expected to be slightly more

intense than K Å 6, and the K Å 1 lines which are all split by asymmetry (while K Å 3 lines are not) were expected to be somewhat weaker than K Å 6 lines. No spin–rotation splittings were expected or observed as none are found in the normal species. The approach taken to the analysis was to pick out series in the P- and R-branch portions of the spectrum and then to use the estimated rotational constants to join them. Three strong series were readily recognized and joined between the P- and R-branch regions. These were expected to correspond to K Å 0, 2, and 4. The observation of a Q-branch for one of these series determined that it corresponded to K Å 4. Another Q-branch was observed which led to the assignment of a fifth series corresponding to K Å 6. However, no apparent Q-branches could be found for K Å 3 or K Å 2. (The Q-branches for K Å 1 would clearly be too weak to observe.) At high N, one of the two remaining series begins to split into two equal intensity components; it was assigned as K Å 2 with the splitting being the result of asymmetry making the assignment of the remaining strong series perforce correspond to K Å 0. We have not been able to assign K Å 3 or K Å 1 even though the K Å 3 P- and Rbranches are expected to be stronger than the corresponding assigned K Å 6 lines. We will discuss below further efforts to assign K Å 3. Tables 1 through 4 list the observed transition frequencies of the assigned lines. Many lines remain unassigned that

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FIG. 3. Q-branch of the K Å 4 subband. Although K Å 4 is perturbed, the perturbations are found at N § 14. Thus only the last line observed, N Å 14, is affected by a small perturbation.

could be due to the odd K levels as well as to hot bands from low-lying vibrational states, and there are several discernible short series in both the P- and R-branch regions which have so far not been linked. Some of these unassigned lines are obvious in Fig. 2. The upper state energy levels are perturbed. Therefore, ground state rotational constants were determined by fitting combination differences using Watson’s A-reduced Hamiltonian. Because the K Å 1 lines were not assigned and only parallel lines were assigned, A 9, B 9 0 C 9, d N9 , and d K9 are heavily correlated. The data determine the symmetric rotor 9 . The determination quantities, (B 9 / C 9 )/2, D N9 , and D NK of A 9 and D K9 requires DK x 0 transitions which are not present. The effects of asymmetry are manifested most obviously in the K Å 2 asymmetry splittings which arise through second order perturbation and depend primarily upon ((B 9 0 C 9 )/4) 2 /(A 9 0 (B 9 / C 9 )/2). Thus only B 9 / C 9, D N9 , D NK 9 , and ((B 9 0 C 9 )/4) 2 /(A 9 0 (B 9 / C 9 )/2) were fitted. These ground state constants are given in Table 5. The standard deviation of the combination difference fit residuals is 0.0009 cm01 . To date, the most advanced theoretical structure for this molecule is that of Botschwina et al. (27). The rotational constants determined from this ab initio geometry are given also given in Table 5 for comparison. The calculated values of the average of the B and C constants, which is our best determined constant, match our experimental numbers to within 0.4%. The upper state energy levels were determined by adding the calculated lower state energy to the observed frequency.

However, global fitting the upper state proved impossible because of perturbations. The only K subband which appears to be unperturbed is K Å 6, as K Å 2 is globally shifted and K Å 0 and K Å 4 exhibit local perturbations. As an example, Fig. 4 shows the effect of three perturbative interactions within the K Å 0 subband perturbing the local structure and demonstrates that at least three separate levels are affecting the system. K Å 2 has no local perturbations except at the highest N values (N Å 31–33) assigned, but is globally shifted as mentioned previously. Because of these perturbations, a different approach to fitting the upper state had to be adopted. As a first step, some approach to the treatment of the effects of asymmetry in the upper state had to be devised. The approach used was to fit the asymmetry splittings of the K Å 2 upper state levels. This is not desirable because, as we shall see below, almost every parameter associated with K Å 2 is affected by perturbation. However, there is no alternative, because the K Å 0 and K Å 2 levels are most strongly affected by asymmetry and there is no way to separate the effect of asymmetry from a change in D *N for K Å 0. The value of ((B * 0 C * )/4) 2 /(A * 0 (B * / C * )/2) resulting from fitting the asymmetry splittings of the K Å 2 upper state levels is given in Table 5. The energy levels of each subband were fit with n0 , B *, and D *N , omitting the levels of K Å 0 and K Å 4 near the local perturbations. The results of this fitting are given in Table 6. If the levels were unperturbed, plots of n0 , B *, and D *N versus K 2 should give straight lines. Figure 5 shows a

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B * and D *NK and are listed in Table 5. Figure 7 shows D *N vs K 2 . As might be expected, the higher order quantity D *N is most sensitive to the perturbations, and it is unwise to average the three points with K x 2. Probably the best estimate of D *N is the value found for the almost completely unperturbed K Å 6 subband and that is listed in Table 5. Because the errors are caused by perturbations, it is pointless to attempt to provide statistical uncertainties for the upper state parameters listed in Table 5. A systematic study of approximately 250 unassigned lines was implemented in an effort to identify the two K Å 1 and the K Å 3 subbands. Using the ground state rotational constants determined above, the ground state combination differences for K Å 3 should be accurately predictable. However, in order to calculate K Å 1 ground state combination differences, B–C must be estimated. This was done by assuming that the inertial defect of CH2CCD is the same as that of CH2CCH. The validity of this assumption is questionable. Because of this uncertainty in combination difference prediction for K Å 1 and because the K Å 3 lines are expected to be more intense than the K Å 1 lines, our assignment efforts focused primarily on K Å 3. A computer program was written that compared all Pand R-branch region differences to all those predicted for K

TABLE 1 Observed K Å 0 R 0 Frequencies (cm01 )

TABLE 2 Observed K Å 2 R 2 Frequencies (cm01 )

plot of the origins of the K subbands versus K 2 . The K Å 2 level has been pushed down approximately 2.5 cm01 from the position expected from a smooth curve through the other origins. Such a displacement could be caused by either Fermi resonance or Pa-type Coriolis interaction. This global perturbation ensures that the upper state cannot be fit with the simple Hamiltonian used to fit the lower state. The K Å 2 point was omitted and the remaining three points fitted with a quadratic to determine n0 , D A, and D( DK ). The results are given in Table 5. Note that the value of D( DK ) is almost as large as the ground state value of DK . This is rather physically unrealistic result probably is caused by global perturbations of K Å 0 and/or K Å 4 that are smaller than the global perturbation of K Å 2. Figure 6 shows the same kind of plot of an effective B * vs K 2 . The effective B *s were fitted by a straight line, omitting the K Å 2 point, to obtain Copyright q 1997 by Academic Press

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are expected to be small. Here B * and B 9 are the effective B values for K Å 3; e.g., B 9 (K Å 3) Å B 9 0 9DNK . The approximate value of the constant a can be estimated, but the reliability of the estimate is limited by the information that the K Å 2 origin is strongly perturbed and thus the K Å 3 origin might also be perturbed. The constant b can be estimated from B * 0 B 9 from the other K values, and the constant c should be small. It thus seems likely that K Å 3 can be assigned by plotting the many values of [P(N * / 1) / R(N * 0 1)]/2 versus N * (N * / 1). When we attempted this, we found four apparently possible assignments. However, none of these possibilities were convincing; i.e., attempts to extend these assignments to other lines were not deemed sufficiently successful.

TABLE 3 Observed K Å 4 R 4 Frequencies (cm01 )

4. DISCUSSION

In the normal species (CH2CCH), the K structure is not resolved at low N. Thus, there is a single dominant series which runs through the spectrum consisting of overlapped and unresolved K components where all but the K Å 1 bands

TABLE 4 Observed K Å 6 R 6 Frequencies (cm01 )

Å 3 and listed all matches within experimental error. For each match there is a possible assignment of the upper state N value, N *. For each such match, the average of the two lines P(N * / 1) and R(N * 0 1) was calculated. It is easy to show that, if the upper state K Å 3 levels are unperturbed, for the correct assignments a plot of these averages versus N * (N * / 1) should be a quadratic

P(N * / 1) / R(N * 0 1) 2

[1]

Å a / bN * (N * / 1) / c[N * (N * / 1)]

2

with a Å £0 (K Å 3) 0 B 9 / 2D 9N / acorr

[2]

b Å B * 0 B 9 / 6D 9N / bcorr

[3]

c Å 0 (D *N 0 D 9N ) / ccorr ,

[4]

where the correction terms arise from the asymmetry and Copyright q 1997 by Academic Press

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TABLE 5 Rotational Constants

overlap. In contrast, for CH2CCD, the K structure of the CD stretch is very well resolved. At higher N in the normal species, the series belonging to different K gradually split apart. In the previously published work on this species (31), no K assignments were made. Recently, we (40) have as-

signed the K series for K Å 0, 1, 2, 3, 4, 6 of CH2CCH by using ground state combination differences calculated from the rotational constants of Tanaka (34). The acetylenic CH stretch of CH2CCH is remarkably unperturbed with only one major (local) perturbation (in K Å 2) observed. In contrast, there are many perturbations both local and global in the acetylenic CD stretch of CH2CCD. Using the fundamental frequencies of the symmetric modes and the harmonic frequencies of the nonsymmetric modes from the ab initio calculation of Botschwina (27), we calculate that the vibrational density of states in the CD stretch region is approximately one per cm01 . This makes it difficult to assign the perturbing states. The global perturbation of K Å 2 represents the best chance for assigning the perturbing state, because it is large enough to suggest that perhaps only a small number of vibrational modes are excited in the perturbing level. The K Å 2 perturbation can only be either a purely vibrational interaction or an a-type Coriolis interaction, because all the different N levels observed are shifted by the same amount. A b- or c-type Coriolis interaction would have interaction matrix elements of q the form N(N / 1) 0 K(K / 1) , leading to an N-dependent shift in the levels. For a purely vibrational interaction, the perturbing level must be of symmetry A1 . Considering first that the perturbing level has two quanta of vibrational excitation, there is no combination of two modes of symmetry B1 or two modes of symmetry B2 that is even remotely possible, and only one combination with two symmetric modes singly excited that

FIG. 4. Local perturbations in the K Å 0 subband.

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TABLE 6 Parameters Resulting from Fitting Individual K-Subbranches (cm01 )

is fairly close, n4 / n5 , predicted by Botschwina at 2487 cm01 . This prediction is well below the origin frequency determined here of 2557 cm01 , but considering that the experimental value (34) of n6 (687.176 cm01 ) for the undeuterated species is 43 cm01 higher than the ab initio value (27) (644 cm01 ) this seems within the realm of possibility. In order for only K Å 2 to be significantly affected, but not K Å 0 or K Å 4, the value of the A rotational constant should be quite different in the perturbing state from that of the CD stretch. This is plausible since n4 is the CH2 bend. If excitation of three quanta of vibration in the perturbing level is permitted, more possibilities open up. The closest two (neglecting anharmonicity associated with n7 and n12 and the anharmonic interaction between n7 and n12 with n3 ) are n3 / 2n7 , predicted at 2568 cm01 , and n3 / 2n12 , at 2544 cm01 . Other possibilities, assuming a larger uncertainty in the ab initio values and anharmonicity effects, are n4 / n6 / n8 (2493 cm01 ) and n3 / n7 / n8 (2607 cm01 ). An a-type Coriolis interaction would require that the perturbing level be of symmetry A2 , since the level being perturbed is of A1 symmetry. There is no possible combination of states involving only two singly excited modes with one

FIG. 5. K-subband origins n0 plotted versus K 2 . These origins are expected to follow the equation n(K) Å n0 / ( D A)K 2 0 D( DK )K 4 . Obviously there is a perturbation for K Å 2.

necessarily of b1 symmetry and the other necessarily of b2 symmetry using Botschwina’s frequencies. Again there are several possibilities if three quanta are excited in the perturbing level. Since n2 is of A1 symmetry, n7 is B1 , and n12 is B2 , the cases discussed above immediately suggest n2 / n7 / n12 predicted at 2556 cm01 as a possible a-type Coriolis perturber. However, any a-type Coriolis interaction with the CD stretch should be very small because this vibration is primarily on the a-axis and along the a-axis. The HBr lines seen in Fig. 2 indicate that the 193 nm photons used to photolyze the propargyl bromide may be creating trace amounts of d-propargylene (HCCCD). We would like to verify the time behavior of this signal to determine whether propargylene is indeed a primary product of the flash photolysis or whether the HBr might be generated in some other way, but this was not feasible with the experimental setup employed. 5. CONCLUSION

The spectrum of the CD stretch of CH2 CCD has been assigned for even K values, with several hundred transitions being identified. The lower state constants determinable

FIG. 6. K-subband B values versus K 2 . The expected behavior is Beff ( K) Å B 0 DNK K 2 . Again there is a clear perturbation for K Å 2.

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FIG. 7. DN K-subband values versus K 2 . There is no clear pattern here, presumably because this quantity is sensitive to the local perturbations in the K Å 0 and K Å 4 subbands as well as to the global perturbation in the K Å 2 subband.

from these assignments have been obtained. Although the upper state levels are extensively perturbed, approximate parameters have been extracted. The information thus gained should be of use in future kinetics work.

mL, 0.24 mol) and equipped with an overhead stirrer. Two dropping addition funnels were installed. One dropping funnel was charged with bromine (11.8 mL, 0.23 mol) and the other with a solution of pyridine (19.3 mL, 0.24 mol) and 3-d-propyn-1-d-ol (12.64 g, 0.22 mol). Note. pyridine and propargyl alcohol generate heat upon mixing. Efficient stirring was maintained during all additions. Bromine was introduced over 15 min while the flask was periodically cooled in an ice water bath to keep the reaction below room temperature. The mixture was stirred for 10 min after all the bromine had been added. The pyridine/propynol solution was then added over 15 min with cooling as required. After complete addition, the reaction was stirred for 3 h at room temperature. The addition funnels were removed and one of the necks stoppered. The crude bromide was removed in vacuo (2.0 Torr) and condensed in a cold trap submerged in dry ice/acetone. The contents of the trap were fractionally distilled (12.5 cm glass helices) to provide 3-d-propargyl bromide (20.05 g, 77%, b.p. 407C/140 torr). The material was stored over MgO. ACKNOWLEDGMENTS This work was supported by the National Science Foundation and the Robert A. Welch Foundation. WCE was supported by a Department of Defense National Defense Science and Engineering Graduate Fellowship. The authors thank Professor Peter Botschwina for helpful correspondence.

APPENDIX: SYNTHESIS OF 3-d-PROPARGYL BROMIDE

General. Ether refers to diethyl ether. Ether, propargyl alcohol, and pyridine were distilled before use. An inert atmosphere was maintained during all reactions and distillations. 3-d-Propyn-1-d-ol. To a stirred solution of K2CO3 (8.0 gm, 0.058 mol, anhydrous) in D2O (50 mL, 2.76 mol) at room temperature was added propargyl alcohol (15 mL, 0.26 mol) via syringe over 1 min. The progress of the equilibration was monitored by 1H NMR (250 MHz, D2O). After two days, 25 mL of ether was added to the stirred solution. Solid NaCl was then added in portions until the aqueous phase was saturated. The mixture was then transferred to a 1 L separatory funnel for extraction with ether (5 1 100 mL). The combined organic layers were dried with MgSO4 , filtered, and distilled (24/40 30 cm Vigreaux) down to about 25 mL. Removal of solvent in vacuo is more convenient but results in slightly lowered yields. The residue was transferred with dry ether rinsings to a 100 mL roundbottom flask and fractionally distilled (14/20 12.5 cm Vigreaux) to yield 3-d-propyn-1-d-ol (12.64 gm, 0.217 mol, 84%, b.p. 1147C). Deuterium incorporation at C-3 was 91% based on normalized 1H NMR integration ratios of hydrogen on C-3 and C-1. 3-d-Propargyl Bromide. A three-necked 500-mL roundbottomed flask was charged with triphenyl phosphite (62.8

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