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JOURNAL OF RAMAN SPECTROSCOPYJ. Raman Spectrosc. 2002; 33: 443–448Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jrs.862

Investigation of resonantly selected Raman spectra ofintermediates in organic pyrolysis reactions

Hiroshi Matsui,1∗ Nadya Kobko,1 J. J. Dannenberg,1 Seth H. Jonas2 and Raji Viswanathan3

1 Department of Chemistry, Hunter College of the City University of New York, New York, New York 10021, USA2 Department of Physics, University of Central Florida, Orlando, Florida 32816, USA3 Department of Chemistry, Yeshiva University, New York, New York 10033, USA

Received 17 July 2001; Accepted 25 January 2002

The usefulness of UV resonance Raman (UVRR) spectroscopy in determining intermediate configurationsin pyrolysis reactions was examined. We investigated the conformations of the nitrosoketene, E- andZ-isomers, generated via the pyrolysis of isonitroso Meldrum’s acid by their characteristic vibrationalwavenumbers. In this system, UVRR with 355 nm excitation worked exceptionally well to enhanceselectively the nitrosoketene peaks. The signal was virtually free from other intermediates interferences,which was a major problem in IR spectroscopy. While both E- and Z-nitrosoketene were found as theintermediates of the isonitroso Meldrum’s acid pyrolysis by UVRR, we consider that the observation oftwo isomers is due to their interconversions at 150 ◦C, based upon computational analysis. Therefore,spectroscopic information on a shorter time-scale is necessary to determine the intermediate configurationin pyrolysis reactions. This method will still be useful for monitoring reactions at low temperatures, whichmay not involve significant interconversions of isomers. Copyright 2002 John Wiley & Sons, Ltd.

INTRODUCTION

The detection of isomers in chemical reactions is vital fordetermining the reaction mechanisms. Vibrational spec-troscopy is an effective tool for determining reaction mecha-nisms by analyzing vibrational modes of reactants, productsand intermediates. One of the limitations of spectroscopicmethods for intermediate detection is the low signal levelbecause intermediates are generally formed in trace amounts.In some reactions the intermediate signal level is high, butvarious species in the reactants, products and intermedi-ates may cause overlapping of their peaks in the spectra.This complication in the spectra makes it difficult to deter-mine precise vibrational wavenumbers of the intermediates.Anomalous or interfering signals reduce the resolution ofthe intermediate’s signal, as such signals are generally veryweak. Therefore, when reaction intermediates are analyzedby spectroscopy, the intermediate spectra must be isolated(i.e. free of interference from other species) in order to deter-mine the exact vibrational wavenumbers. If isomers arepresent in a reaction sequence, their vibrational wavenum-bers are in proximity to one another. Serious interference

ŁCorrespondence to: Hiroshi Matsui, Department of Chemistry,Hunter College of the City University of New York, 695 ParkAvenue, New York, New York 10021-5085, USA.E-mail: [email protected]/grant sponsor: Laser Analytics Company.Contract/grant sponsor: National Science Foundation.Contract/grant sponsor: Office of Basic Energy Sciences of theDepartment of Energy.

from other molecules will diminish finely resolved distinc-tions between two isomers. Of course, another significantfactor for intermediate detection is the stability of the inter-mediate conformations. Generally, pyrolysis reactions aremonitored at relatively high temperatures so that the inter-mediates may react with other species or undergo intercon-versions of isomers on a short time-scale, which may preventthe determination of absolute intermediate conformations.Here we only studied the effectiveness of spectroscopic iso-lation since fast time-scale pump–probe experiments are notavailable in our laboratories.

In this work, we studied gas-phase pyrolysis productsof isonitroso Meldrum’s acid (IMA) (Fig. 1). Nitrosoketene(Fig. 1), produced via pyrolysis of IMA, is a very usefulintermediate to generate functional heterocyclic compounds,nitrones.1 While nitrones have been used to synthesize˛-amino acids stereoselectively with various functionalgroups,2,3 it is possible that the conformation of thenitrosoketene, E- and Z-forms (Fig. 1), may affect theconformations of the nitrones, and ultimately those of ˛-amino acids.4 Hence the detection of nitrosoketene isomers inthe pyrolysis reaction is of keen interest in controlling yieldsof amino acid products from intermediate conformations.Ketene is also considered to be an effective catalyst forcertain polymerizations.5

The vibrational wavenumbers of the E- and Z-isomers,computed by RHF/6–31GŁ ab initio calculation (see Table 1),show a maximum wavenumber separation of only 22 cm�1

(in the C C vibration), and a minimum wavenumber

Copyright 2002 John Wiley & Sons, Ltd.

444 H. Matsui et al.

O

O

O

O

N

OH

Me

Me

Isonitroso Meldrum's Acid

N

O

O

C

(Z ) - nitrosoketene

N

OC

O

(E ) - nitrosoketene

Figure 1. Structures of isonitroso Meldrum’s acid andnitrosoketene.

separation of only 16 cm�1 (in the C O vibration). Sucha fine differential between isomers illustrates the need forinterference-free spectra of nitrosoketene. Mechanistic inves-tigation of IMA’s pyrolysis was previously attempted byusing Fourier transform IR spectroscopy.6 Unfortunately,the IR spectra contained series of peak overlaps between theketene and acetone, another intermediate produced via IMApyrolysis. This interference was removed through appro-priate background subtraction, but pertinent informationwas lost since the acetone peaks were dominant in theirintensities. After subtraction of the acetone and CO2 peaks,the ketene peaks showed very weak intensities. Withoutany intensity amplification, the peaks were indiscernible. Itwas possible to amplify the remaining ketene spectrum, butthe low signal-to-noise ratio made it difficult to determinethe exact vibrational wavenumbers of the isomers, E- andZ-forms, in this subtracted IR spectrum.

UV resonance Raman (UVRR) spectroscopy offers a novelapproach to organic intermediate determination. Throughthe resonance effect, we can selectively enhance the signal of aparticular species in the organic reaction. This tuning is basedon the characteristic electronic absorption of a molecule.Since most organic molecules with any level of conjugationreadily absorb UV radiation, UVRR is an appropriate choiceto study the IMA pyrolysis mechanism.

We use UVRR spectroscopy to investigate the confor-mations of nitrosoketene by their characteristic vibrationalwavenumbers. In this system, UVRR worked exceptionallywell owing to the weak Raman intensity of acetone vibra-tions, which minimize its interference with the nitrosoketenespectrum. The nitrosoketene isomers are expected to havelarge Raman cross-sections with UV excitation, and there-fore their Raman intensities will be enhanced well abovethe acetone peaks. Hence the signal is virtually free fromacetone interference and we can analyze the conformationsof the intermediates. UVRR provided a discrete distinctionbetween the spectra of (E)- and (Z)-nitrosoketene.

EXPERIMENTAL

Isonitroso Meldrum’s acid (Fig. 1) was prepared accordingto the published procedure.7 The reaction cell was heated byan Aerorod stainless-steel tubular heater, wound over theentire surface of the cross. Extra heating was needed aroundthe quartz windows to avoid adhesion of the sample to the

windows. Hence the window region was preheated to 90 °Cwith heating tape prior to heating the sample cell. This pre-heating was not sufficient to vaporize the sample in the cell,which was confirmed by Raman measurement. Temperaturewas controlled by a thermistor (Thermometrics), capableof 0.1 °C precision. The heater power was distributedhomogeneously by a layer of aluminum foil held in place byglass tape and wound with spun fiberglass tape as a finalinsulator barrier. The heater and temperature sensor werecoupled using a Hart Scientific Model 2100 controller.

The isonitroso Meldrum’s acid sample (6.94 ð 10�4 mol)in this reaction cell was evacuated to 10�4 Torr (1 Torr D133.3 Pa) by a diffusion pump for 12 h to minimize watervapor in the cell. This reaction cell was then installed inthe UVRR system. The third and fourth harmonics of anNd : YAG laser (Laser Analytics, Model YQL-102) operatingwith 6 ns duration pulses at 10 Hz was the light source ofthis system. The 266 and 355 nm laser beams were focusedon the center of the reaction cell to a spot size of 5 mmdiameter with the pulsed laser energy set at 20 mJ, and theRaman scattering was collected at right-angles to the incidentbeam direction. A monochromator (JY/Horiba Triax 550)dispersed the Raman scattered light, which was detectedby a UV-enhanced CCD detector (JY/Horiba Spectrum-1)and processed by JY/Horiba data acquisition hardwareand software. The isonitroso Meldrum’s acid pyrolysisintermediates were monitored by this experimental settingas a function of temperature.

We calculated the reaction profile between the E- andZ-isomers of nitrosoketene using density functional theorycalculations at the B3LYP/6–311CCGŁŁ level. The calcu-lations were performed using the Gaussian 98 suite ofprograms.8 The B3LYP method combines Becke’s three-parameter functional9 with the non-local correlation pro-vided by the correlation functional of Lee, Yang and Parr.10

The geometries were completely optimized without anyconstants. Standard integration grids were employed. Wecalculated the vibrational wavenumbers of all stationarypoints to characterize them and obtain the H, S and Gvalues. These calculations used the harmonic oscillator, rigidrotor and ideal gas approximations at 150 °C and 1 atm.

The equilibrium geometries of the E- and Z-conformations of nitrosoketene were calculated at therestricted Hartree–Fock (RHF)11 level theory using aDunning–Hay12 double-zeta basis set. Polarization functionsconsisting of a set of p functions on each hydrogenatom and a set of d functions on each carbon atom,and a set of diffuse s functions on the hydrogen atomsand a set of diffuse sp functions on the carbon atomswere also included. The vibrational wavenumbers andthe normal mode displacements were calculated for theground state at the RHF level of theory. All the excited-statecalculations were performed at the equilibrium geometry ofthe ground state since the electronic excitations are expectedto be vertical. The energy and potential gradients of the

Copyright 2002 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2002; 33: 443–448

Resonance Raman spectra of intermediates in organic pyrolysis reactions 445

excited electronic state were calculated using the multi-configurational SCF (CASSCF)13 – 15 method.

In order to obtain the resonance Raman intensitiescorresponding to electronic excitation, the gradient of theenergy with respect to the Cartesian coordinates, obtainedfrom the ab initio calculation, was projected on to the normalmode coordinates:

∂VŁ/∂Qk D �∂VŁ/∂qi��∂qi/∂Qk� �1�

where ∂VŁ/∂qi is the gradient of the excited-state potential incartesian space and ∂qi/∂Qk are the cartesian displacementvectors for the normal mode Qk and the summation is over allcartesian coordinates. The resonance Raman intensities, Ik,corresponding to the normal mode, Qk, are then proportionalto these gradients:

Ik ¾ �1/�k��∂VŁ/∂Qk�2 �2�

where �k is the fundamental vibrational frequency of thenormal mode Qk.16,17 All the ab initio calculations, includingthe normal-mode wavenumbers and displacement vectors,were calculated using the GAMESS18,19 package. All thecomputations were performed on an SGI/O2 system.

RESULTS AND DISCUSSION

Third and fourth harmonic outputs of the Nd : YAG laserwere applied for Raman excitations. The 266 nm excita-tion yielded very weak UV Raman scattering from thenitrosoketene and no spectroscopic structure was observed.With excitation at 355 nm, excellent enhancement of the

Raman spectrum for nitrosoketene was observed [Fig. 2(a)].A neat acetone Raman spectrum with the same 355 nm exci-tation is shown in Fig. 2(c). The strongest peak in the acetonespectrum, 1700 cm�1, was absent in nitrosoketene spectrum[Fig. 2(a)]. Therefore, we conclude that the interference ofacetone in the nitrosoketene spectrum is minimal. This fea-ture increased the signal-to-noise ratio of the nitrosoketenespectrum owing to the lack of interference from acetonepeaks, sufficient to determine the vibrational wavenumbersof the nitrosoketene isomers.

While we recognize that the electronic structures of iso-mers address the degrees of resonance Raman enhancementsbetween the two excitation wavelengths, it is difficult toprepare each isomer for the UV–Vis absorption measure-ment. Installation of the large pyrolysis reaction cell inour UV–Vis spectrometer was also problematic. Therefore,electronic absorptions of (E)- and (Z)-nitrosoketene werecomputed with a Configuration Interaction (CI) of the singlyexcited state for comparison (Fig. 3), although the outcomemay not be very reliable. Based on this calculation, it seemsthat the exceedingly strong absorption of nitrosoketene at266 nm traps the Raman scattering of nitrosoketene that isalso around 266 nm. Excitation at 355 nm, a second maxi-mum located nearly 100 nm away from the first absorptionmaximum, seems to tune appropriately for the detection ofnitrosoketene.

It was found previously that the relative concentrationsof nitrosoketene as a function of temperature were maxi-mum when the pyrolysis took place at 150 °C.6 At roomtemperature, nitrosoketene was not produced from the IMApyrolysis as shown by the lack of the characteristic peaks inthe Raman spectrum in Fig. 2(b), which is consistent with the

Figure 2. Raman spectra of the intermediates in the thermal decomposition of isonitroso Meldrum’s acid at (a) 150 °C and (b) roomtemperature. (c) Raman spectrum of neat acetone at 150 °C.



Figure 3. Computed UV–Vis absorption spectra of(E)-nitrosoketene (A) and (Z)-nitrosoketene (B). Wavelengths oflaser excitations, 266 and 355 nm, are shown by arrows.

previous observation.6 At 150 °C, UVRR resolved indepen-dent peaks, each corresponding to either the E- or Z-isomerin Fig. 2(a). Assignments were made by comparison withvibrational wavenumbers computed at the RHF/6–31GŁ

level. Vibrational wavenumbers were scaled on the basis ofprevious calculations.6 The two peaks at 1318 and 1335 cm�1

are assigned to the C C stretching mode of (Z)- and (E)-nitrosoketene, respectively. A doublet at 1403 and 1417 cm�1

is assigned as N O stretching modes. In the N O peaks,the lower wavenumber corresponds to E-nitrosoketene andthe higher to wavenumber (Z)-nitrosoketene. The peaksat 2141 and 2172 cm�1 are assigned as C O stretchingmodes of (Z)- and (E)-nitrosoketene, respectively. Theseassignments are summarized in Table 1. Computed vibra-tional wavenumbers and resonance Raman intensities of (E)-and (Z)-nitrosoketene were compared with the experimentalUVRR spectrum, as illustrated in Fig. 4. The spectral profilesof the E- and Z-isomers are consistent between the computa-tion and the experiment. Therefore, this result indicates thatboth (E)- and (Z)-nitrosoketene are produced via the IMApyrolysis.

The enthalpy of (Z)-nitrosoketene was calculated as only1.06 kcal mol�1 (1 kcal D 4.184 kJ) above the enthalpy of (E)-nitrosoketene at the B3LYP/6–311CCGŁŁ level, as shown inTable 2. This calculation does not show a strong energeticadvantage in favor of one conformation of nitrosoketenes.

Table 1. Experimental and computed vibrationalwavenumbers of (E)- and (Z)-nitrosoketene at 150 °C

Wavenumbers/cm�1

Conformation Band Experiment RHF/6–31GŁ

Z- C O 2141 2118N O 1417 1402C C 1318 1269

E- C O 2172 2134N O 1403 1385C C 1335 1291

Figure 4. Illustration of the computed Raman spectra for (E)-and (Z)-nitrosoketenes and experimental nitrosoketenespectrum via pyrolysis of isonitroso Meldrum’s acid. Ramanintensities were computed including the resonance effects(electronic states were optimized at the RHF level).

Table 2. Relative energetics of (E)- and (Z)-nitrosoketene andthe transition state in kcal mol�1 at 150 °C

Conformation Energy Enthalpy Free energy

Z- 1.02 1.06 0.71E- 0.00 0.00 0.00Transition state 18.47 17.30 17.27

The activation enthalpy for the isomerization between the E-and Z-forms is 17.30 kcal mol�1. This energetic configurationis summarized in Fig. 5. Using the Boltzmann equation,k D N�kBT/h� exp��G/RT�, we obtain a rate constant of1.52 ð 104 s�1. In this equation, N is the statistical numberof the transition state (Fig. 5) and N D 2 in this casesince there are two ways to isomerize from the E- tothe Z-conformation by turning the N—O group clockwiseor counter-clockwise. kB is the Boltzmann constant andT D 150 °C. This result indicates that the isomers undergorapid interconversion owing to the high value of the rateconstant. From the Boltzmann distribution, the proportion


Resonance Raman spectra of intermediates in organic pyrolysis reactions 447

Figure 5. Illustration of relative energetics of (E)- and(Z)-nitrosoketene and the transition state.

of the Z-conformation is calculated as 30% at equilibriumat 150 °C. In the experiment, no changes in the Ramanintensities of the isomers were observed over several hours.This observation suggests that the interconversion of theisomers is equilibrated at 150 °C, which is consistent withour computed result above.

The Raman spectrum also contains some unassignedprominent peaks, indicated by double asterisks in Fig. 2(a).Since nitrosoketene has been observed to be dimerized as[2 C 2] and/or [3 C 2] forms (illustrated in Table 3),6 wecomputed the vibrational wavenumbers of these dimersfor comparison with the unassigned peaks in Fig. 2(a). Thevibrational wavenumbers of these dimers were calculated atthe RHF/6–31GŁ level, as summarized in Table 3. These

Table 3. Computed vibrational wavenumbers(cm�1) of nitrosoketene dimers

RHF/6–31GŁ

O

O

ON

NO

[2+2] dimer

N

O

N

O

OH

H O

[3+2] dimer

C O — 1931C O 1930 1788C N 1087 1659N ! O 1757 1538

vibrational wavenumbers were scaled on the basis ofprevious calculations.6 The peak at 1511 cm�1 is assignedas the N—O stretching mode in the [3 C 2] dimer. The peakat 1636 cm�1 is assigned as the C N stretch for the [3 C 2]dimer. There are no peaks matching the calculated [2 C 2]dimer vibrational wavenumbers, but this may be due toweak Raman activities of [2 C 2] dimer vibrations. Therefore,some portions of the nitrosoketene molecules seem to bedimerized, even in the diluted gaseous state.

CONCLUSION

UVRR spectroscopy was used to investigate the conforma-tions of the nitrosoketene generated via the pyrolysis ofisonitroso Meldrum’s acid by their characteristic vibrationwavenumbers. In this system, UVRR with 355 nm excita-tion worked exceptionally well to enhance selectively thenitrosoketene peaks. The signal was virtually free fromacetone interference, which was a major problem in IR spec-troscopy. UVRR provided a discrete distinction between the(E)- and (Z)-nitrosoketene spectra. According to computa-tions, the observation of two isomers is probably due to theinterconversion of the E- and Z-isomers and therefore thisoutcome does not determine the absolute intermediate con-formations. This result demonstrated that UVRR is usefulin isolating certain intermediate spectra, but shorter time-scale UVRR will be necessary for the detection of pyrolysisintermediates owing to their instability at high temperatures.

AcknowledgementsThis work was supported by Laser Analytics Company, the NationalScience Foundation and the Office of Basic Energy Sciences of theDepartment of Energy. H.M. thanks Professor Edward R. Grant forthe use of his reaction cell and Professor Nobuya Katagiri for sampledonations. We appreciate helpful discussions with Professor DavidM. Birney.

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