Radical channel photodissociation dynamics of aliphatic aldehydes: the nascent state distribution of the HCO photoproduct

Terentis, Andrew C.; Knepp, Pamela T.; Kable, Scott H.

doi:10.1117/12.220865

Cited by 4 publications

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“…The LIF spectrum of nascent HCO was measured after exciting propanal at 30 649 cm -1 which corresponds to the first moderately large peak in the phofex spectrum (marked with an asterisk in the inset to Figure ). Transitions were observed arising from only the ground vibrational state of HCO, despite searching for the appearance of known hot-bands. ,− We concentrated our attention on the origin transition, B̃ (0,0,0) ← X̃ (0,0,0), which has an appearance similar to HCO spectra that have been published previously following dissociation of propanal, propenal, acetaldehyde, , and formaldehyde. − The spectrum is complex because of the number of rotational branches and the multiple splittings caused by spin-rotation and similar asymmetry. Figure shows a small region of the HCO spectrum to indicate the typical congestion prevalent throughout the entire spectrum.…”

Section: Resultsmentioning

confidence: 99%

“…More recently, we have investigated the photodissociation dynamics of propanal at 309.1 nm by monitoring the nascent HCO population distribution of N and K a states via LIF spectroscopy. , The population distribution of N and K a states could be reasonably characterized by a Boltzmann distribution with a temperature of 480 ± 50 K corresponding to an average rotational energy of 6.0 ± 0.6 kJ mol -1 . An examination of the Doppler profiles of a number of transitions led to the conclusion that the majority of excess energy was partitioned into HCO and ethyl translational motion, the average being 23 ± 4 kJ mol -1 , and that the ethyl fragment contains very little internal energy.…”

Section: Introductionmentioning

confidence: 99%

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Near Threshold Photochemistry of Propanal. Barrier Height, Transition State Structure, and Product State Distributions for the HCO Channel

2002

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The photodissociation dynamics of propanal have been investigated at photolysis wavelengths between 300 and 327 nm. The threshold for production of HCO fragments was found to be 326.26 nm, which corresponds to 30645 cm-1 (366.6 kJ mol-1) above the zero-point of the S 0 state. From known thermochemical data, this threshold lies 25.0 ± 3.6 kJ mol-1 above the bond dissociation energy. The nascent HCO rotational and translational energy distributions were determined following dissociation at threshold. The rotational population was measured as a function of N, K a, K c, and S. The distribution of rotational states followed a Gaussian function with an average rotational energy of 2.5 ± 0.5 kJ mol-1. The population of the near-degenerate spin-rotation states was equal, while the population in the asymmetry doublets favored the upper energy component by about 3:1. Careful measurement of the Doppler profiles of individual K a = 0 lines in the LIF spectrum revealed that the translational energy also shows a Gaussian-like distribution with an average energy of 6.5 ± 1.0 kJ mol-1. The ethyl fragment must also have an average translational energy of 6.5 ± 1.0 kJ mol-1 and therefore an average internal energy of 9.5 kJ mol-1 is inferred. The observed energy partitioning in the fragments is consistent with a model in which the HCO rotational and translational excitation is determined mostly by the transition state geometry, a barrier on the triplet surface, and the fixed energy in the exit channel. A modified impulsive model was satisfactory in reproducing the energy deposited into the various degrees of freedom. The model implied impact parameters at infinite separation corresponding to an in-plane HCO angle of 40° and an out-of-plane angle of 60°. The strongly pyramidal nature of the transition state produces more angular momentum about the b axis than the c axis, which causes the preference for the upper energy component of the asymmetry doublets.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Near Threshold Photochemistry of Propanal. Barrier Height, Transition State Structure, and Product State Distributions for the HCO Channel

2002

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“…At 193 nm, the methoxy (CH 3 O) radical represents the dominant product, whereas hydroxymethyl (CH 2 OH) and methyl (CH 3 ) radicals are formed predominantly at 157 nm . Photodissociation of ethanol (CH 3 CH 2 OH) and n -propyl alcohol (CH 3 CH 2 CH 2 OH) follows a similar pattern, such as the generation of the ethoxy (CH 3 CH 2 O) photofragment. , The photodissociation of aldehydes (RCHO, R = CH 3 (CH 2 ) n ; n = 0–2) was also explored, and C–H and C–C bond rupture were determined to be the major decomposition pathways leading to RCO and HCO radicals, respectively. − Secondary dissociation of these fragments via, for example, atomic hydrogen loss leading to (CH 2 ) n +1 CO ( n = 0–2) and carbon monoxide (CO), respectively, strongly depend on the internal energy redistribution prior to bond breaking with the yield of the secondary products increasing as the photon energy rises. In contrast to aforementioned systems, photodissociation of nitriles (R-CN, R = CH 3 (CH 2 ) n ; n = 0–2) is less diverse and exhibits predominantly the carbon–carbon cleavage to the cyano radical (CN) plus the alkyl radical fragment (R). , The quantum yield decreases with the length of the alkyl chain due to intramolecular vibrational energy redistribution (IVR).…”

Section: Introductionmentioning

confidence: 99%

Photodissociation Dynamics of Astrophysically Relevant Propyl Derivatives (C₃H₇X; X = CN, OH, HCO) at 157 nm Exploiting an Ultracompact Velocity Map Imaging Spectrometer: The (Iso)Propyl Channel

et al. 2022

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The photodissociation dynamics of astrophysically relevant propyl derivatives (C3H7X; X = CN, OH, HCO) at 157 nm exploiting an ultracompact velocity map imaging (UVMIS) setup has been reported. The successful operation of UVMIS allowed the exploration of the 157 nm photodissociation of six (iso)propyl systemsn/i-propyl cyanide (C3H7CN), n/i-propyl alcohol (C3H7OH), and (iso)butanal (C3H7CHO)to explore the C3H7 loss channel. The distinct center-of-mass translational energy distributions for the i-C3H7X (X= CN, OH, HCO) could be explained through preferential excitation of the low frequency C–H bending modes of the formyl moiety compared to the higher frequency stretching of the cyano and hydroxy moieties. Although the ionization energy of the n-C3H7 radical exceeds the energy of a 157 nm photon, C3H7 + was observed in the n-C3H7X (X = CN, OH, HCO) systems as a result of photoionization of vibrationally “hot” n-C3H7 fragments, photoionization of i-C3H7 after a hydrogen shift in vibrationally “hot” n-C3H7 radicals, and/or two-photon ionization. Our experiments reveal that at least the isopropyl radical (i-C3H7) and possibly the normal propyl radical (n-C3H7) should be present in the interstellar medium and hence searched for by radio telescopes.

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“…Rate constants for OH radical reactions with C 1 −C 5 aldehydes have been reported previously. − The photolysis of formaldehyde (CH 2 O) and acetaldehyde (CH 3 CHO) has been studied extensively. − Previous studies on the photodecomposition of propionaldehyde (C 2 H 5 CHO, propanal) have been carried out at a few irradiation wavelengths in the actinic UV region, − and the peak radical yield reported by the same group 9,10 differed by almost a factor of 4. Recently, Terentis and co-workers , reported the nascent state distribution of the HCO photoproduct from the 308 and 309 nm photolysis of propionaldehyde, but they did not obtain the HCO radical yields. Determination of the wavelength-dependent photolysis quantum yields of propionaldehyde allows a comparison with those reported previously for formaldehyde and acetaldehyde 5-8 and with our recent results on C 5 aldehydes. , It also permits an estimation of atmospheric radical formation rate constants from propionaldehyde photolysis.…”

Section: Introductionmentioning

confidence: 99%

The Wavelength Dependence of the Photodissociation of Propionaldehyde in the 280−330 nm Region

Chen

Zhu

2001

J. Phys. Chem. A

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We have investigated the photodecomposition of propionaldehyde (C2H5CHO; propanal) at 5 nm intervals in the 280−330 nm region by using dye laser photolysis combined with cavity ring-down spectroscopy. Absorption cross sections were determined for propionaldehyde. The HCO radical was a fragment from photodissociation. The HCO radical yields, obtained by monitoring its transient absorption at 613.8 nm, decreased with increasing C2H5CHO pressure in the 1−10 Torr range due to the increasing HCO + HCO, HCO + C2H5, and HCO + C2H5CHO reactions at higher propionaldehyde pressures and quenching by ground state propionaldehyde. After separating the contribution of HCO radical reactions, the propionaldehyde pressure quenching effect was only observed at photolysis wavelengths longer than 315 nm. Values of zero-pressure HCO yields (all λ) and ratios of quenching to unimolecular decay rate constant of excited propionaldehyde (λ ≥ 315 nm) were given. The HCO yields (φHCO°) were 0.98 ± 0.06, 0.92 ± 0.06, 0.95 ± 0.08, 0.98 ± 0.11, 0.91 ± 0.05, and 1.08 ± 0.07 at 295, 300, 305, 310, 315, and 320 nm, indicating that C2H5CHO + hν → C2H5 + HCO is the dominant photolysis pathway. The HCO yields decreased at both the shorter-wavelength (280 nm) and the longer-wavelength (330 nm) ends. The wavelength dependence of the HCO yields from propionaldehyde photolysis was compared to that from t-pentanal ((CH3)3CCHO) photolysis. The HCO yields from t-pentanal photolysis decayed much more rapidly at the shorter-wavelength end, which might reveal the difference in the excited states singlet−triplet surface crossing of t-pentanal versus propionaldehyde. The dependence of the HCO yields on nitrogen buffer gas pressure was examined between 10 and 400 Torr. No dependence was observed. Cross section results were combined with HCO radical yields to estimate atmospheric photodissociation rate constants of propionaldehyde to form HCO as a function of zenith angle for cloudless conditions and at 760 Torr nitrogen pressure. Radical formation rate constants were 1.6 × 10-5 − 4.6 × 10-5 s-1 for zenith angles of 0−60°.

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Radical channel photodissociation dynamics of aliphatic aldehydes: the nascent state distribution of the HCO photoproduct

Cited by 4 publications

References 0 publications

Near Threshold Photochemistry of Propanal. Barrier Height, Transition State Structure, and Product State Distributions for the HCO Channel

Near Threshold Photochemistry of Propanal. Barrier Height, Transition State Structure, and Product State Distributions for the HCO Channel

Photodissociation Dynamics of Astrophysically Relevant Propyl Derivatives (C₃H₇X; X = CN, OH, HCO) at 157 nm Exploiting an Ultracompact Velocity Map Imaging Spectrometer: The (Iso)Propyl Channel

The Wavelength Dependence of the Photodissociation of Propionaldehyde in the 280−330 nm Region

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Radical channel photodissociation dynamics of aliphatic aldehydes: the nascent state distribution of the HCO photoproduct

Cited by 4 publications

References 0 publications

Near Threshold Photochemistry of Propanal. Barrier Height, Transition State Structure, and Product State Distributions for the HCO Channel

Near Threshold Photochemistry of Propanal. Barrier Height, Transition State Structure, and Product State Distributions for the HCO Channel

Photodissociation Dynamics of Astrophysically Relevant Propyl Derivatives (C3H7X; X = CN, OH, HCO) at 157 nm Exploiting an Ultracompact Velocity Map Imaging Spectrometer: The (Iso)Propyl Channel

The Wavelength Dependence of the Photodissociation of Propionaldehyde in the 280−330 nm Region

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Product

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Photodissociation Dynamics of Astrophysically Relevant Propyl Derivatives (C₃H₇X; X = CN, OH, HCO) at 157 nm Exploiting an Ultracompact Velocity Map Imaging Spectrometer: The (Iso)Propyl Channel