Structural dynamics effects on the electronic predissociation of alkyl iodides

Abstract

The correlation between chemical structure and predissociation dynamics has been evaluated for a series of linear and branched alkyl iodides with increasing structural complexity by means of femtosecond time-resolved velocity map imaging experiments following excitation on the second absorption band (B-band) at around 201 nm. The time-resolved images for the iodine fragment are reported and analyzed in order to extract electronic predissociation lifetimes and the temporal evolution of the anisotropy while the experimental results are supported by ab initio calculations of the potential energy curves as a function of the C-I distance. Remarkable similarities are observed for all molecules consistent with a major predissociation of the initially populated bound Rydberg states 6A″ and 7A′ through a crossing with the purely repulsive states 7A″, 8A′ and 8A″ leading to a major R + i*(2 P 1/2) (R = cH 3 , c 2 H 5 , n-c 3 H 7 , n-c 4 H 9 , i-c 3 H 7 and t-c 4 H 9) dissociation channel. The reported electronic predissociation lifetimes are found to decrease for an increasing size of the linear radical, reflecting the shifts observed in the position of the crossings in the potential energy curves, and very likely a greater non-adiabatic coupling between the initially populated Rydberg states and the repulsive states leading to dissociation induced by other coordinates associated to key vibrational normal modes. The loss of anisotropy is fully accounted for by the parent molecular rotation during predissociation and the rotational temperature of the parent molecule in the molecular beam is reasonably derived. Time-resolved femtosecond pump-probe experiments supported by ab initio calculations have been largely reported in recent years for a variety of molecular systems, aiming at a deeper understanding of the nuclear and electronic dynamics occurring in different photodissociation and photoionization processes 1. Among them, the correlation between increasing structural complexity of alkyl iodides and the ultrafast C-I bond cleavage was investigated by our group employing femtosecond time-resolved velocity map imaging (VMI) 2 in conjunction with ab initio full-dimension time-resolved dynamics calculations. For this purpose, a series of linear and branched alkyl iodide molecules were excited at 266 nm in the first absorption band (A-band) which arises from the n(5p,I) → σ * (C-I) transition 3. Several repulsive electronic states can be populated leading to dissociation mediated by a conical intersection either into the major channel R + I * (2 P 1/2) or into the minor one R + I(2 P 3/2)henceforth denoted as I * and I, respectively. Reported reaction times for both channels I * and I showed to steadily increase with increasing R radical size. The experimental and theoretical findings were rationalized by a 1-D model correlating the reaction time τ, the reduced mass of the molecule and an energy variable, E' = E av-E int , accounting for the difference between the total available energy, E av , and the internal energy of the ...

Ion-Pair Formation in n-Butyl Bromide through 5p Ryberg State Predissociation

“…22 The energy for the ion-pair of [CH 3 + −Br − ] was found to be 9.5 and 8.15 eV for [C 2 H 5 + −Br − ], 27,43 demonstrating that larger alkyl groups stabilize the ion-pair state. 19 We calculate the ion-pair energy for n-butyl bromide to be 7.1 eV, consistent with the trend of ion-pair formation energy of alkyl halides decreasing with carbon chain length. It should be noted that none of the Rydberg states are dissociative, but all states above the s Rydberg states contain sufficient energy to enable ion-pair formation.…”

“…The energy required to form the ion-pair is found via E false( normalA + , normalB − false) = D 0 ( A B + ) + IP ( A B ) − E normalA ( B ) where D 0 is the dissociation energy of the cation species, IP is for the parent molecule, and E A is for the anion . The energy for the ion-pair of [CH 3 + –Br – ] was found to be 9.5 and 8.15 eV for [C 2 H 5 + –Br – ], , demonstrating that larger alkyl groups stabilize the ion-pair state . We calculate the ion-pair energy for n -butyl bromide to be 7.1 eV, consistent with the trend of ion-pair formation energy of alkyl halides decreasing with carbon chain length.…”

“…18 Long alkyl structures inefficiently convert excess energy to the translational motion of photofragments and therefore exhibit increased internal excitation with alkyl size. 18,19 At energies higher than the A state exists a series of bound Rydberg states. In alkyl halides, exciting to the 5s Rydberg (often labeled B and C) states causes the molecule to undergo internal conversion into the repulsive A state on the order of ∼1.5 ps.…”

“…At energies higher than the A state exists a series of bound Rydberg states. In alkyl halides, exciting to the 5s Rydberg (often labeled B and C) states causes the molecule to undergo internal conversion into the repulsive A state on the order of ∼1.5 ps. ,− Internal conversion to the A state depends on the length of the alkyl chain as the intersection of the A and B states in longer alkyl chains occurs closer to the FC region, leading to faster crossing times with increasing carbon chain length . The A state dissociation times are also dependent on the structure with branched isomers having shorter time scales for dissociation.…”

Nonstatistical Photoinduced Processes in Gaseous Organic Molecules

“…As much as the combined body of data presented in this review takes this aspiration one step closer to reality, we are still not quite there yet. The actual direct observations of ultrafast changes in bonding environment are limited, with the only clear-cut relation being a structural motif and an associated ultrafast bond breakage that is found for halides . In this case, the R–X bond is so sufficiently weak that the n → σ* state is low enough in energy to be accessed by a UV photon .…”