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Context.—: Photobleaching artifact occurs when fluorescence intensity decreases following light exposure. Slides stained with fluorescent techniques may be stored in the dark until primary diagnostics. Experimental evidence suggesting the rate of photobleaching and necessity of dark storage is lacking., Objective.—: To compare photobleaching rate on direct immunofluorescence and Thioflavin T slides stored in ambient room light conditions and exposed to excitatory wavelengths., Design.—: During 2 iterations of the experiment, 45 slides were prepared, 42 with immunofluorescent antibodies plus 3 with thioflavin, from skin and kidney biopsies. The experimental group was stored in room light conditions in comparison to the control in the dark, at room temperature. Further, 1 immunofluorescence slide and 1 thioflavin slide were exposed to excitatory fluorescent light for several hours. Significant photobleaching was defined as an integer decrease in score (scale, 0-3)., Results.—: Exposure times ranged from 152 to 3034 hours. Nine of the 42 immunofluorescence slides (21%) photobleached after a minimum exposure of 152 hours to room light, with no significant difference between the experimental and control groups (all P values >.05). The immunofluorescence slide exposed to fluorescent light for 4 hours showed marked photobleaching in the exposed field but not elsewhere. No thioflavin slides showed clinically significant photobleaching under any conditions., Conclusions.—: Clinically significant photobleaching of slides exposed to room light may occur after a few days, but not a few hours (unless exposed to excitatory fluorescent light). Conversely, thioflavin-stained slides did not photobleach when exposed to ambient room air and photobleached only negligibly when exposed to excitatory fluorescent light., Competing Interests: The authors have no relevant financial interest in the products or companies described in this article., (© 2024 College of American Pathologists.)

Despite their prevalence in biomass and importance in biochemistry, there is still much to be learned about simple carbohydrates. Gas-phase calculations are reported here on two trioses and three tetroses. For aldotetroses, both the open-chain and furanose forms are considered. Enthalpies of reduction to polyols are calculated at the CBS-APNO level of theory, and comparisons to simple aldehydes and ketones are made. Heats of formation are calculated in two ways with overall good agreement. The heat of formation of glyceraldehyde obtained from modified HEAT calculations is also reported. Finally, calculated bond energies are presented, and the influence of the structure on the bond energies is discussed.

The unique optical cycling efficiency of alkaline earth metal-ligand molecules has enabled significant advances in polyatomic laser cooling and trapping. Rotational spectroscopy is an ideal tool for probing the molecular properties that underpin optical cycling, thereby elucidating the design principles for expanding the chemical diversity and scope of these platforms for quantum science. We present a comprehensive study of the structure and electronic properties in alkaline earth metal acetylides with high-resolution microwave spectra of 17 isotopologues of MgCCH, CaCCH, and SrCCH in their 2 Σ + ground electronic states. The precise semiexperimental equilibrium geometry of each species has been derived by correcting the measured rotational constants for electronic and zero-point vibrational contributions calculated with high-level quantum chemistry methods. The well-resolved hyperfine structure associated with the 1,2 H, 13 C, and metal nuclear spins provides further information on the distribution and hybridization of the metal-centered, optically active unpaired electron. Together, these measurements allow us to correlate trends in chemical bonding and structure with the electronic properties that promote efficient optical cycling essential to next-generation experiments in precision measurement and quantum control of complex polyatomic molecules.

The highly successful velocity map imaging (VMI) technique plays a central role in revealing light-matter interactions. Here we demonstrate the related but distinct kinetic energy-broadened spatial map imaging (KESMI) option for recovering KE and angular recoil information on photophysical processes using a VMI system operating in different out-of-focus modes. The characteristic single or double stripes and related steps in the vertical intensity profiles of KESMIs of photoelectrons (PEs) from Ar ionization allow breakthrough developments of a potent global model that enables an understanding and analysis of these patterns. These signatures reflect the relationship between the observed features and predicted convolved discrete KEs and angular distributions. The derivation of the velocity distribution of the PEs ensuing from the ionization of a single H 2 O quantum state based on the measured and simulated KESMI provides another rigorous test demonstrating and realizing the feasibility of this new approach, which holds future promise on its own or combined with VMI.

The high resolution far-infrared spectrum of trans -butadiene has been reinvestigated by Fourier-transform spectroscopy at two synchrotron radiation facilities, SOLEIL and the Canadian Light Source, at temperatures ranging from 50 to 340 K. Beyond the well-studied bands, two new fundamental bands lying below 1100 cm -1 , ν 10 and ν 24 , have been assigned using a combination of cross-correlation (ASAP software) and Loomis-Wood type (LWWa software) diagrams. While the ν 24 analysis was rather straightforward, ν 10 exhibits obvious signs of a strong perturbation, presumably owing to interaction with the dark ν 9 + ν 12 state. Effective rotational constants have been derived for both the v 10 = 1 and v 24 = 1 states. Since only one weak, infrared active fundamental band (ν 23 ) of trans -butadiene remains to be observed at high resolution in the far-infrared, searches for the elusive gauche conformer can now be undertaken with considerably greater confidence in the dense ro-vibrational spectrum of the trans form.

The 193-nm photolysis of CH 2 CHCN illustrates the capability of chirped-pulse Fourier transform millimeter-wave spectroscopy to characterize transition states. We investigate the HCN, HNC photofragments in highly excited vibrational states using both frequency and intensity information. Measured relative intensities of J = 1-0 rotational transition lines yield vibrational-level population distributions (VPD). These VPDs encode the properties of the parent molecule transition state at which the fragment molecule was born. A Poisson distribution formalism, based on the generalized Franck-Condon principle, is proposed as a framework for extracting information about the transition-state structure from the observed VPD. We employ the isotopologue CH 2 CDCN to disentangle the unimolecular 3-center DCN elimination mechanism from other pathways to HCN. Our experimental results reveal a previously unknown transition state that we tentatively associate with the HCN eliminated via a secondary, bimolecular reaction., Competing Interests: The authors declare no competing interest.

Surprising features in a recently published high-level calculation of the rotational profile of butadiene led us to compare butadiene with a set of 17 heterodienes. The rotational profiles for this large group of compounds varied widely, thereby possessing a high information content. These data were subjected to a Fourier analysis yielding 1- through 6-fold terms: the one-fold terms represent the change in steric energy on going from 180° to 0°, while the changes in the 2-fold terms correspond to the expected change in π-delocalization energy with structure; the 3-fold terms were significant and found to be linearly correlated to the average of the atomic charges of the atoms at the central single bond of the cis-forms, but their origins are still not clear; we propose a novel 1,4 π-interactions that may account for this phenomenon. The 4-fold terms were at times comparable in magnitude to the 3-fold terms but overall appeared to mainly modify the 3-fold terms slightly without introducing any qualitatively new features. The 5- and 6-fold terms were negligible.

The nascent steps in the pyrolysis of the lignin components salicylaldehyde ( o-HOC 6 H 4 CHO) and catechol ( o-HOC 6 H 4 OH) were studied in a set of heated microreactors. The microreactors are small (roughly 1 mm ID × 3 cm long); transit times through the reactors are about 100 μs. Temperatures in the microreactors can be as high as 1600 K, and pressures are typically a few hundred torr. The products of pyrolysis are identified by a combination of photoionization mass spectrometry, photoelectron photoion concidence mass spectroscopy, and matrix isolation infrared spectroscopy. The main pathway by which salicylaldehyde decomposes is a concerted fragmentation: o-HOC 6 H 4 CHO (+ M) → H 2 + CO + C 5 H 4 ═C═O (fulveneketene). At temperatures above 1300 K, fulveneketene loses CO to yield a mixture of HC≡C-C≡C-CH 3 , HC≡C-CH 2 -C≡CH, and HC≡C-CH═C═CH 2 . These alkynes decompose to a mixture of radicals (HC≡C-C≡C-CH 2 and HC≡C-CH-C≡CH) and H atoms. H-atom chain reactions convert salicylaldehyde to phenol: o-HOC 6 H 4 CHO + H → C 6 H 5 OH + CO + H. Catechol has similar chemistry to salicylaldehyde. Electrocyclic fragmentation produces water and fulveneketene: o-HOC 6 H 4 OH (+ M) → H 2 O + C 5 H 4 ═C═O. These findings have implications for the pyrolysis of lignin itself.

The adiabatic ionization energy of hydrogen peroxide (HOOH) is investigated, both by means of theoretical calculations and theoretically assisted reanalysis of previous experimental data. Values obtained by three different approaches: 10.638 ± 0.012 eV (purely theoretical determination), 10.649 ± 0.005 eV (reanalysis of photoelectron spectrum), and 10.645 ± 0.010 eV (reanalysis of photoionization spectrum) are in excellent mutual agreement. Further refinement of the latter two values to account for asymmetry of the rotational profile of the photoionization origin band leads to a reduction of 0.007 ± 0.006 eV, which tends to bring them into even closer alignment with the purely theoretical value. Detailed analysis of this fundamental quantity by the Active Thermochemical Tables approach, using the present results and extant literature, gives a final estimate of 10.641 ± 0.006 eV.

We report the combination of tabletop vacuum ultraviolet photoionization with photoion-photoelectron coincidence spectroscopy for sensitive, isomer-specific detection of nascent products from a pyrolysis microreactor. Results on several molecules demonstrate two essential capabilities that are very straightforward to implement: the ability to differentiate isomers and the ability to distinguish thermal products from dissociative ionization. Here, vacuum ultraviolet light is derived from a commercial tabletop femtosecond laser system, allowing data to be collected at 10 kHz; this high repetition rate is critical for coincidence techniques. The photoion-photoelectron coincidence spectrometer uses the momentum of the ion to identify dissociative ionization events and coincidence techniques to provide a photoelectron spectrum specific to each mass, which is used to distinguish different isomers. We have used this spectrometer to detect the pyrolysis products that result from the thermal cracking of acetaldehyde, cyclohexene, and 2-butanol. The photoion-photoelectron spectrometer can detect and identify organic radicals and reactive intermediates that result from pyrolysis. Direct comparison of laboratory and synchrotron data illustrates the advantages and potential of this approach.

Two methyl esters were examined as models for the pyrolysis of biofuels. Dilute samples (0.06-0.13%) of methyl acetate (CH 3 COOCH 3 ) and methyl butanoate (CH 3 CH 2 CH 2 COOCH 3 ) were entrained in (He, Ar) carrier gas and decomposed in a set of flash-pyrolysis microreactors. The pyrolysis products resulting from the methyl esters were detected and identified by vacuum ultraviolet photoionization mass spectrometry. Complementary product identification was provided by matrix infrared absorption spectroscopy. Pyrolysis pressures in the pulsed microreactor were about 20 Torr and residence times through the reactors were roughly 25-150 μs. Reactor temperatures of 300-1600 K were explored. Decomposition of CH 3 COOCH 3 commences at 1000 K, and the initial products are (CH 2 ═C═O and CH 3 OH). As the microreactor is heated to 1300 K, a mixture of CH 2 ═C═O and CH 3 OH, CH 3 , CH 2 ═O, H, CO, and CO 2 appears. The thermal cracking of CH 3 CH 2 CH 2 COOCH 3 begins at 800 K with the formation of CH 3 CH 2 CH═C═O and CH 3 OH. By 1300 K, the pyrolysis of methyl butanoate yields a complex mixture of CH 3 CH 2 CH═C═O, CH 3 OH, CH 3 , CH 2 ═O, CO, CO 2 , CH 3 CH═CH 2 , CH 2 CHCH 2 , CH 2 ═C═CH 2 , HCCCH 2 , CH 2 ═C═C═O, CH 2 ═CH 2 , HC≡CH, and CH 2 ═C═O. On the basis of the results from the thermal cracking of methyl acetate and methyl butanoate, we predict several important decomposition channels for the pyrolysis of fatty acid methyl esters, R-CH 2 -COOCH 3 . The lowest-energy fragmentation will be a 4-center elimination of methanol to form the ketene RCH═C═O. At higher temperatures, concerted fragmentation to radicals will ensue to produce a mixture of species: (RCH 2 + CO 2 + CH 3 ) and (RCH 2 + CO + CH 2 ═O + H). Thermal cracking of the β C-C bond of the methyl ester will generate the radicals (R and H) as well as CH 2 ═C═O + CH 2 ═O. The thermochemistry of methyl acetate and its fragmentation products were obtained via the Active Thermochemical Tables (ATcT) approach, resulting in Δ f H 298 (CH 3 COOCH 3 ) = -98.7 ± 0.2 kcal mol -1 , Δ f H 298 (CH 3 CO 2 ) = -45.7 ± 0.3 kcal mol -1 , and Δ f H 298 (COOCH 3 ) = -38.3 ± 0.4 kcal mol -1 .

Both glycolaldehyde and glyoxal were pyrolyzed in a set of flash-pyrolysis microreactors. The pyrolysis products resulting from CHO-CH2OH and HCO-CHO were detected and identified by vacuum ultraviolet (VUV) photoionization mass spectrometry. Complementary product identification was provided by argon matrix infrared absorption spectroscopy. Pyrolysis pressures in the microreactor were about 100 Torr, and contact times with the microreactors were roughly 100 μs. At 1200 K, the products of glycolaldehyde pyrolysis are H atoms, CO, CH2═O, CH2═C═O, and HCO-CHO. Thermal decomposition of HCO-CHO was studied with pulsed 118.2 nm photoionization mass spectrometry and matrix infrared absorption. Under these conditions, glyoxal undergoes pyrolysis to H atoms and CO. Tunable VUV photoionization mass spectrometry provides a lower bound for the ionization energy (IE)(CHO-CH2OH) ≥ 9.95 ± 0.05 eV. The gas-phase heat of formation of glycolaldehyde was established by a sequence of calorimetric experiments. The experimental result is ΔfH298(CHO-CH2OH) = -75.8 ± 1.3 kcal mol(-1). Fully ab initio, coupled cluster calculations predict ΔfH0(CHO-CH2OH) of -73.1 ± 0.5 kcal mol(-1) and ΔfH298(CHO-CH2OH) of -76.1 ± 0.5 kcal mol(-1). The coupled-cluster singles doubles and noniterative triples correction calculations also lead to a revision of the geometry of CHO-CH2OH. We find that the O-H bond length differs substantially from earlier experimental estimates, due to unusual zero-point contributions to the moments of inertia.

The thermal decomposition of cyclohexanone (C6H10═O) has been studied in a set of flash-pyrolysis microreactors. Decomposition of the ketone was observed when dilute samples of C6H10═O were heated to 1200 K in a continuous flow microreactor. Pyrolysis products were detected and identified by tunable VUV photoionization mass spectroscopy and by photoionization appearance thresholds. Complementary product identification was provided by matrix infrared absorption spectroscopy. Pyrolysis pressures were roughly 100 Torr, and contact times with the microreactors were roughly 100 μs. Thermal cracking of cyclohexanone appeared to result from a variety of competing pathways, all of which open roughly simultaneously. Isomerization of cyclohexanone to the enol, cyclohexen-1-ol (C6H9OH), is followed by retro-Diels-Alder cleavage to CH2═CH2 and CH2═C(OH)-CH═CH2. Further isomerization of CH2═C(OH)-CH═CH2 to methyl vinyl ketone (CH3CO-CH═CH2, MVK) was also observed. Photoionization spectra identified both enols, C6H9OH and CH2═C(OH)-CH═CH2, and the ionization threshold of C6H9OH was measured to be 8.2 ± 0.1 eV. Coupled cluster electronic structure calculations were used to establish the energetics of MVK. The heats of formation of MVK and its enol were calculated to be ΔfH298(cis-CH3CO-CH═CH2) = -26.1 ± 0.5 kcal mol(-1) and ΔfH298(s-cis-1-CH2═C(OH)-CH═CH2) = -13.7 ± 0.5 kcal mol(-1). The reaction enthalpy ΔrxnH298(C6H10═O → CH2═CH2 + s-cis-1-CH2═C(OH)-CH═CH2) is 53 ± 1 kcal mol(-1) and ΔrxnH298(C6H10═O → CH2═CH2 + cis-CH3CO-CH═CH2) is 41 ± 1 kcal mol(-1). At 1200 K, the products of cyclohexanone pyrolysis were found to be C6H9OH, CH2═C(OH)-CH═CH2, MVK, CH2CHCH2, CO, CH2═C═O, CH3, CH2═C═CH2, CH2═CH-CH═CH2, CH2═CHCH2CH3, CH2═CH2, and HC≡CH.

The value for the HCN → HNC 0 K isomerization energy has been investigated by combining state-of-the-art electronic structure methods with the Active Thermochemical Tables (ATcT) approach. The directly computed energy difference between HCN and HNC at the HEAT-456QP level of theory is 5236 ± 50 cm(-1). This is substantially lower (by ∼470 cm(-1) or ∼1.3 kcal/mol) than the recently proposed high-level multireference configuration interaction value of 5705 ± 20 cm(-1) of Barber et al. ( Mon. Not. R. Astron. Soc. 2014, 437, 1828-1835 ). The discrepancy was analyzed by the ATcT approach, using several distinct steps, which (a) independently corroborated the current single-reference HEAT-456QP result, (b) independently found that the recent multireference-based value is highly unlikely to be correct within its originally stated uncertainty, and (c) produced a recommended value of 5212 ± 30 cm(-1) for the HCN → HNC isomerization energy at 0 K, based on all currently available knowledge. The ATcT standard enthalpies of formation at 0 and 298 K for HCN, HNC, and their cations and anions are also presented.

We report the discovery in space of a disilicon species, SiCSi, from observations between 80 and 350 GHz with the IRAM 30m radio telescope. Owing to the close coordination between laboratory experiments and astrophysics, 112 lines have now been detected in the carbon-rich star CW Leo. The derived frequencies yield improved rotational and centrifugal distortion constants up to sixth order. From the line profiles and interferometric maps with the Submillimeter Array, the bulk of the SiCSi emission arises from a region of 6″ in radius. The derived abundance is comparable to that of SiC 2 . As expected from chemical equilibrium calculations, SiCSi and SiC 2 are the most abundant species harboring a Si-C bond in the dust formation zone and certainly both play a key role in the formation of SiC dust grains.

The acetylene emission spectrum from the trans-bent electronically excited à state to the linear ground electronic X̃ state has attracted considerable attention because it grants Franck–Condon access to local bending vibrational levels of the X̃ state with large-amplitude motion along the acetylene ⇌ vinylidene isomerization coordinate. For emission from the ground vibrational level of the à state, there is a simplifying set of Franck–Condon propensity rules that gives rise to only one zero-order bright state per conserved vibrational polyad of the X̃ state. Unfortunately, when the upper level involves excitation in the highly admixed ungerade bending modes, ν4′ and ν6′, the simplifying Franck–Condon propensity rule breaks down--as long as the usual polar basis (with v and l quantum numbers) is used to describe the degenerate bending vibrations of the X̃ state--and the intrapolyad intensities result from complicated interference patterns between many zero-order bright states. In this article, we show that, when the degenerate bending levels are instead treated in the Cartesian two-dimensional harmonic oscillator basis (with vx and vy quantum numbers), the propensity for only one zero-order bright state (in the Cartesian basis) is restored, and the intrapolyad intensities are simple to model, as long as corrections are made for anharmonic interactions. As a result of trans ⇌ cis isomerization in the à state, intrapolyad emission patterns from overtones of ν4′ and ν6′ evolve as quanta of trans bend (ν3′) are added, so the emission intensities are not only relevant to the ground-state acetylene ⇌ vinylidene isomerization, they are also a direct reporter of isomerization in the electronically excited state.

In the first half of this study, rotational and vibrational constants of six Franck-Condon bright vibrational levels of S1 doubly-substituted (13)C acetylene are determined from laser-induced fluorescence spectra and an updated geometry of the trans conformer of S1 acetylene is obtained. In the second half, we determine the quadratic force field of S1 acetylene on the basis of the harmonic frequencies of four isotopologues of acetylene. The effects of both diagonal and off-diagonal x(ij) anharmonicities are removed from the input harmonic frequencies. Results from both experimental and theoretical studies of various isotopologues of acetylene (including those from the first half of this paper) are used to obtain a set of force constants that agrees well with ab initio calculations. Our set of force constants for S1 acetylene is an improvement over previous work by Tobiason et al., which did not include off-diagonal anharmonicities.

Highly accurate calculations are reported for properties of vinylidene (H2C═C:), specifically the position of its zero-point vibrational level relative to that of acetylene and its equilibrium structure and ground state rotational constants. The isomerization energy of vinylidene calculated at the HEAT-456QP level of theory is 43.53 ± 0.15 kcal mol(-1), in agreement with the previous best estimate, but associated with a much smaller uncertainty. In addition, the thermochemical calculations presented here also allow a determination of the H2CC-H bond energy of the vinyl radical at the HEAT-345(Q) level of theory, which is 77.7 ± 0.3 kcal mol(-1). The equilibrium structure of vinylidene, estimated with an additivity scheme that includes treatment of correlation effects beyond CCSD(T) as well as relativistic and adiabatic (diagonal Born-Oppenheimer correction) contributions, is rCC = 1.2982 ± 0.0003 Å, rCH = 1.0844 ± 0.0003 Å, and θCCH = 120.05 ± 0.05°, with zero-point rotational constants (including vibrational contributions and electronic contributions to the moment of inertia) estimated to be A0 = 9.4925 ± 0.0150 cm(-1), B0 = 1.3217 ± 0.0017 cm(-1), and C0 = 1.1602 ± 0.0016 cm(-1).