Umbrales auditivos en pacientes con enfermedad de Méniere manejados con corticoides transtimpánicos / Hearing thresholds in Ménière's disease patients managed with transtympanic dexametasone

Introducción: Pese a que el uso de corticoides transtimpánicos en pacientes con enfermedad de Méniere es habitual en muchos centros, la evidencia respecto de su efecto sobre los umbrales auditivos es aún controversial. Objetivo: Estudiar los umbrales auditivos de pacientes con enfermedad de Méniere que recibieron corticoides transtimpánicos en el Servicio de Otorrinolaringología del Hospital Clínico de la Universidad de Chile. Material y Método: Estudio retrospectivo de pacientes con enfermedad de Méniere que consultaron entre los años 2015 y 2021. Se estudiaron los umbrales auditivos, antes y después de 3 inyecciones de dexametasona transtimpánica. Resultados: Se obtuvieron datos completos de 27 pacientes. Al comparar el promedio tonal puro antes y después del tratamiento, no se observaron diferencias significativas. A nivel individual, la variación de cambio de los umbrales auditivos con dexametasona se correlaciona en forma significativa con los umbrales auditivos previos a las inyecciones y con el tiempo transcurrido desde la última inyección, pero no con la edad. Conclusión: La terapia con dexametasona transtimpánica en pacientes con enfermedad de Méniere no altera los umbrales auditivos. Sin embargo, se requieren más estudios, para comprobar, si existe un efecto transitorio en los umbrales auditivos de los primeros días posterior al procedimiento.

Introduction: Although transtympanic corticosteroids are proposed in Méniere's disease patients refractory to standard medical therapy, the evidence regarding the effect of transtympanic corticosteroids on hearing thresholds is still controversial. Aim: To study the hearing thresholds of patients with Méniere's disease who were administrated with transtympanic corticosteroids at the Otorhinolaryngology Service of the University of Chile's Clinical Hospital. Material and Method: Retrospective study of Méniere's disease patients who consulted between 2015 and 2021. Demographic variables and hearing thresholds were studied before and after three transtympanic injections of dexamethasone. Results: A total of 27 patients were studied. There were non-significant differences in pure-tone hearing threshold averages before and after the injections. Individual variation in hearing thresholds correlates significantly with the pre-injection hearing thresholds and the period since the last injection, but not with age. Conclusion: Transtympanic dexamethasone therapy in patients with Meniere's disease does not alter hearing thresholds. However, more studies are needed to verify whether there is a transitory effect on hearing thresholds in the first days after the procedure.

Are singleton pregnancies after assisted reproduction technology (ART) associated with a higher risk of placental anomalies compared with non-ART singleton pregnancies? A systematic review and meta-analysis.

BACKGROUND: Placental anomalies (placenta praevia, placental abruption, morbidly adherent placenta and cord insertion anomalies) are associated with maternal and fetal morbidity and mortality. It has been suggested these might be more prevalent in pregnancies after assisted reproduction technology (ART). OBJECTIVES: To determine whether ART singleton pregnancies are associated with an increased risk of placental anomalies compared with non-ART singleton pregnancies. SEARCH STRATEGY: MEDLINE, EMBASE, CENTRAL, Web of Science and Scopus (January 2018). SELECTION CRITERIA: Cohort studies reporting placental anomalies in ART and non-ART singleton pregnancies. DATA COLLECTION AND ANALYSIS: We report pooled odds ratios (OR) for the comparisons: (1) ART versus SC (spontaneously conceived), (2) ART versus non-ART (unspecified), (3) FET-ART (frozen-embryo transfer) versus SC, (4) ART versus non-ART (subfertile patients). Study quality was assessed using a modified Newcastle -Ottawa scale. MAIN RESULTS: 33 low/moderate quality studies evaluated 124 215 ART and 6 054 729 non-ART singleton pregnancies. Risk of placenta praevia, placental abruption and morbidly adherent placenta was higher in ART than SC pregnancies: odds ratio (OR) (OR 3.76, 95% CI 3.09-4.59); (OR 1.87, 95% CI 1.70-2.06) and (OR 2.27, 95% CI 1.79-2.87) respectively. Risk of placenta praevia and placental abruption was higher in ART than in non-ART (subfertile patients): (OR 2.51, 95% CI 2.12-2.98) and (OR 1.61, 95% CI 1.33-1.95) respectively. Results were similar when comparing ART with unspecified non-ART pregnancies. Risk of placenta praevia was higher, but not significantly so, in FET-ART than in SC pregnancies (OR 2.42, 95% CI 0.63-9.30). CONCLUSIONS: Singleton ART pregnancies are associated with an increased risk of placental anomalies compared with non-ART singleton pregnancies. TWEETABLE ABSTRACT: A review of over 6 million singleton pregnancies finds increased risk of placental anomalies after ART.

Application of radiopaque micro-particle fillers for 3-D imaging of periodontal pocket analogues using cone beam CT.

BACKGROUND: Periodontitis is an infectious/inflammatory disease most often diagnosed by deepening of the gingival sulcus, which leads to periodontal pockets (PPs) conventional manual periodontal probing does not provide detailed information on the three-dimensional (3-D) nature of PPs. OBJECTIVES: To determine whether accurate 3-D analyses of the depths and volumes of calibrated PP analogues (PPAs) can be obtained by conventional cone beam computed tomography (CBCT) coupled with novel radiopaque micro-particle fillers (described in the companion paper) injected into the PPAs. METHODS: Two PPA models were employed: (1) a human skull model with artificial gingiva applied to teeth with alveolar bone loss and calibrated PPAs, and (2) a pig jaw model with alveolar bone loss and surgically-induced PPAs The PPAs were filled with controlled amounts of radiopaque micro-particle filler using volumetric pipetting Inter-method and intra-method agreement tests were then used to compare the PPA depths and volumes obtained from CBCT images with values obtained by masked examiners using calibrated manual methods. RESULTS: Significant inter-method agreement (0.938-0.991) and intra-method agreement (0.94-0.99) were obtained when comparing analog manual data to digital CBCT measurements enabled by the radiopaque filler. SIGNIFICANCE: CBCT imaging with radiopaque micro-particle fillers is a plausible means of visualizing and digitally assessing the depths, volumes, and 3-D shapes of PPs This approach could transform the diagnosis and treatment planning of periodontal disease, with particular initial utility in complex cases Efforts to confirm the clinical practicality of these fillers are currently in progress.

Development of radiopaque, biocompatible, antimicrobial, micro-particle fillers for micro-CT imaging of simulated periodontal pockets.

OBJECTIVES: Approximately 109 bacteria can be harbored within periodontal pockets (PP) along with inflammatory byproducts implicated in the pathophysiology of systemic diseases linked to periodontitis (PD). Calculation of this inflammatory burden has involved estimation of total pocket surface area using analog data from conventional periodontal probing which is unable to determine the three-dimensional (3-D) nature of PP. The goals of this study are to determine the radiopacity, biocompatibility, and antimicrobial activity of transient micro-particle fillers in vitro and demonstrate their capability for 3-D imaging of artificial PP (U.S. Patent publication number: 9814791 B2). METHODS: Relative radiopacity values of various metal oxide fillers were obtained from conventional radiography and micro-computed tomography (µCT) using in vitro models. 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays were used to measure the biocompatibility of calcium tungstate (CaWO4) particles by determination of viable keratinocytes percentage (%) after exposure. After introducing an antibacterial compound (K21) to the radiopaque agent, antimicrobial tests were conducted using Porphyromonas gingivalis (P. gingivalis) and Streptococcus gordonii (S. gordonii) strains and blood agar plates. RESULTS: CaWO4 micro-particle-bearing fillers exhibited an X-ray radiopacity distinct from tooth structures that enabled 3-D visualization of an artificial periodontal pocket created around a human tooth. MTT assays indicated that CaWO4 micro-particles are highly biocompatible (increasing the viability of exposed keratinocytes). Radiopaque micro-particle fillers combined with K21 showed significant antimicrobial activity for P. gingivalis and S. gordonii. SIGNIFICANCE: The plausibility of visualizing PP with 3-D radiographic imaging using new radiopaque, biocompatible, transient fillers was demonstrated in vitro. Antibacterial (or other) agents added to this formula could provide beneficial therapeutic features along with the diagnostic utility.

Development of a survivorship needs assessment planning tool for head and neck cancer survivors and their caregivers: a preliminary study.

PURPOSE: The purpose of this study was to characterize primary end-of-treatment challenges in head and neck cancer (HNC) to drive the development of a survivorship needs assessment planning (SNAP) tool and evaluate its acceptability and feasibility. METHODS: Using qualitative methods (focus groups, interviews), we identified physical, emotional, and social post-treatment challenges from the perspectives of survivors (N = 17), caregivers (N = 14), and healthcare providers (N = 14) and pretested the SNAP tool. After Advisory Board ratings and consensus, the tool was finalized. RESULTS: Survivors, caregivers and clinicians consistently highlighted the importance of assessing symptoms and functional abilities (e.g., dry mouth, speech/swallowing difficulties, weight loss), health behaviors (e.g., smoking, alcohol), emotional concerns (e.g., depression, isolation, nutritional distress), and social challenges (e.g., support, finances). Caregivers were overwhelmed and intensely focused on survivors' nutrition and trach/feeding tube care while clinicians emphasized financial and access concerns. Most participants were enthusiastic about the tool and directed a flexible care plan design due to variability in dyad needs. Over 75% reported high comfort using and navigating questions on a tablet and were in strong agreement that the care plan would help families practically and emotionally. Coordination of survivorship visits with follow-up care was critical to address travel and time barriers. While survivors and clinicians recommended waiting 1-6 months after treatment, caregivers preferred earlier survivorship visits. CONCLUSIONS: Results pinpointed optimal end-of-treatment domains for routine assessment and support the feasibility of implementing a SNAP tool in the clinic. IMPLICATIONS FOR CANCER SURVIVORS: Capitalizing on technology to direct HNC survivorship care is promising.

Prospective multicenter cohort study of estrogen and insulin-like growth factor system in BRCA mutation carriers.

PURPOSE: Major clinical characteristics of BRCA1/2-related cancers include association with estrogen and metabolic consequences. We aimed to evaluate serum estradiol (E2) and insulin-like growth factor 1 (IGF-1) levels as a marker of insulin resistance in BRCA1/2 mutation carriers and high-risk, BRCA-negative controls. METHODS: Eligible cancer-free women (age 18-42 with regular menstrual cycles) who had been screened for BRCA1/2 mutations between 2005 and 2013 completed a questionnaire and underwent a single blood draw. E2 was measured with radioimmunoassay, and IGF-1 was measured with enzyme-linked immunosorbent assay. RESULTS: Eighty-six women participated (44 carriers and 42 non-carriers) in this study. BRCA mutation carriers were significantly younger than non-carriers (p = 0.0002). Age-adjusted basal (menstrual cycle days 2-5) serum E2 level was not significantly different between BRCA mutation carriers and non-carriers (30.4 vs. 24.7 pg/mL, p = 0.07). BRCA mutation carriers have significantly lower age-adjusted serum IGF-1 levels compared to non-carriers (89.7 vs. 112.6 ng/mL, p < 0.001). In women with BRCA mutations, the risk of having low serum IGF-1 level (IGF-1: ≤85 ng/mL) was 10.7 times as great as that of women without BRCA mutations (95 % CI 2.5, 46.2). There was a significant inverse association between basal E2 and IGF-1 levels in BRCA mutation carriers after adjusting age and BMI (p = 0.03). CONCLUSIONS: IGF-1 level is significantly lower in cancer-free BRCA mutation carriers versus BRCA-negative controls, and there is a potential association between E2 and IGF-1 in cancer-free BRCA mutation carriers. Our findings may instigate future studies evaluating the role of both E2 and IGF-1 in BRCA mutation carriers.

Unusual presentation of extranodal diffuse large B-cell lymphoma in the head and neck: description of a case with emphasis on radiographic features and review of the literature.

A very unusual radiographic presentation of non-Hodgkin lymphoma (NHL) involving the maxilla is described. The patient was initially managed with antibiotics prescribed to treat what was thought to represent an odontogenic infection. After unsuccessful antibiotic therapy, the patient was referred to an oral surgery clinic where CBCT was performed. CBCT revealed an atypical generalized sclerosis of the affected bone rather than the usual lytic radiographic pattern associated with NHL. Destruction of the sinus floor with infiltration of the sinus was also present. This rare radio-opaque radiographic presentation is described in detail together with the clinical presentation and histopathological findings. The important radiographic features suggesting malignancy that were present in this atypical case of NHL are discussed. A differential diagnosis highlighting the differences between NHL, osteomyelitis and osteosarcoma is also provided.

Imaging of indeterminate pancreatic cystic lesions: a systematic review.

BACKGROUND: Pancreatic cystic lesions are an increasing problem and investigation of these cysts can be fraught with difficulty. There is currently no gold standard for diagnosis or surveillance. This review was undertaken to determine the present reliability of the characterisation, assessment of malignant potential and diagnosis of pancreatic cystic lesions using available imaging modalities. METHODS: A Medline search using the terms 'pancreatic', 'pancreas', 'cyst', 'cystic', 'lesions', 'imaging', 'PET'. 'CT', 'MRI' and 'EUS' was performed. Publications were screened to include studies examining the performance of CT, MRI, MRCP, EUS and 18-FDG PET in the determination of benign or malignant cysts, cyst morphology and specific diagnoses. RESULTS: Nineteen studies were identified that met the inclusion criteria. 18-FDG PET had a sensitivity and specificity of 57.0-94.0% and 65.0-97.0% and an accuracy of 94% in determining benign versus malignant cysts. CT had a sensitivity and specificity of 36.3-71.4% and 63.9-100% in determining benign disease but had an accuracy of making a specific diagnosis of 39.0-44.7%. MRI had a sensitivity and specificity of 91.4-100.0% and 89.7% in assessing main pancreatic duct communication. CONCLUSION: CT is a good quality initial investigation to be used in conjunction with clinical data. MRCP can add useful information regarding MPD communication but should be used judiciously. PET may have a role in equivocal cases to determine malignancy. Further examination of CT-PET in this patient group is warranted.

Computational investigation of the pyrolysis product selectivity for α-hydroxy phenethyl phenyl ether and phenethyl phenyl ether: analysis of substituent effects and reactant conformer selection.

Using computational methods, we determine product selectivities for the pyrolysis of two model compounds for the ß-O-4 linkage in lignin: phenethyl phenyl ether (PPE) and α-hydroxy phenethyl phenyl ether (α-hydroxy PPE). We investigate the dependence of the product selectivities on the number of reactant conformers included. Utilizing density functional theory in combination with transition state theory, we obtain rate constants for hydrogen abstraction reactions by the key chain-carrying radicals, which determine the product selectivity within a steady-state kinetic model. The inclusion of the energetically second lowest reactant conformer of PPE and α-hydroxy PPE has a large effect on the product selectivity. The final product selectivity computed for PPE agrees well with experiment. We find that the α-hydroxy substituent affects energetic as well as entropic contributions to the rate constant differently for alternative pathways of hydrogen abstraction and, thereby, significantly alters product distributions.

Role of carbon-carbon phenyl migration in the pyrolysis mechanism of ß-O-4 lignin model compounds: phenethyl phenyl ether and α-hydroxy phenethyl phenyl ether.

We investigate phenyl shift and subsequent ß-scission reactions for PhCHXCH·OPh [X = H, OH], which are part of the pyrolysis mechanism of phenethyl phenyl ether (PPE) and α-hydroxy PPE. PPE and its derivatives are model compounds for the most common linkage in lignin, the ß-O-4 linkage. We use density functional theory to locate transition states and equilibrium structures and kinetic Monte Carlo in combination with transition-state theory for kinetic simulations. Oxygen-carbon and carbon-carbon phenyl shift reactions proceed through cyclic intermediates with similar barriers. However, while subsequent ß-scission of the oxygen-carbon shift products proceeds with virtually no barrier, the activation energy for ß-scission of the carbon-carbon shift products exceeds 15 kcal/mol. We found that about 15% of ß-radical conversion can be attributed to carbon-carbon shift for PPE and α-hydroxy PPE at 618 K. Whereas the oxygen-carbon shift reaction has been established as an integral part of the pyrolysis mechanism of PPE and its derivatives, participation of the carbon-carbon shift reaction has not been shown previously.

Computational study of bond dissociation enthalpies for substituted ß-O-4 lignin model compounds.

The biopolymer lignin is a potential source of valuable chemicals. Phenethyl phenyl ether (PPE) is representative of the dominant ß-O-4 ether linkage. DFT is used to calculate the Boltzmann-weighted carbon-oxygen and carbon-carbon bond dissociation enthalpies (BDEs) of substituted PPE. These values are important for understanding lignin decomposition. Exclusion of all conformers that have distributions of less than 5% at 298 K impacts the BDE by less than 1 kcal mol(-1). We find that aliphatic hydroxyl/methylhydroxyl substituents introduce only small changes to the BDEs (0-3 kcal mol(-1)). Substitution on the phenyl ring at the ortho position substantially lowers the C-O BDE, except in combination with the hydroxyl/methylhydroxyl substituents, for which the effect of methoxy substitution is reduced by hydrogen bonding. Hydrogen bonding between the aliphatic substituents and the ether oxygen in the PPE derivatives has a significant influence on the BDE. CCSD(T)-calculated BDEs and hydrogen-bond strengths of ortho-substituted anisoles, when compared with M06-2X values, confirm that the latter method is sufficient to describe the molecules studied and provide an important benchmark for lignin model compounds.

Pyrolysis of phenethyl phenyl ether tethered in mesoporous silica. Effects of confinement and surface spacer molecules on product selectivity.

There has been expanding interest in exploring porous metal oxides as a confining environment for organic molecules resulting in altered chemical and physical properties including chemical transformations. In this paper, we examine the pyrolysis behavior of phenethyl phenyl ether (PPE) confined in mesoporous silica by covalent tethers to the pore walls as a function of tether density and the presence of cotethered surface spacer molecules of varying structure (biphenyl, naphthyl, octyl, and hexadecyl). The PPE pyrolysis product selectivity, which is determined by two competitive free-radical pathways cycling through the two aliphatic radical intermediates (PhCH·CH(2)OPh and PhCH(2)CH·OPh), is shown to be significantly different from that measured in the liquid phase as well as for PPE tethered to the exterior surface of nonporous silica nanoparticles. Tailoring the pore surface with spacer molecules further alters the selectivity such that the PPE reaction channel involving a molecular rearrangement (O-C phenyl shift in PhCH(2)CH·OPh), which accounts for 25% of the products in the liquid phase, can be virtually eliminated under pore confinement conditions. The origin of this change in selectivity is discussed in the context of steric constraints on the rearrangement path inside the pores, surface and pore confinement effects, pore surface curvature, and hydrogen bonding of PPE with residual surface silanols supplemented by nitrogen physisorption data and molecular dynamics simulations.

Kinetic analysis of the phenyl-shift reaction in ß-O-4 lignin model compounds: a computational study.

The phenyl-shift reaction for the ß-radical of phenethyl phenyl ether (PhCH(2)CHOPh, ß-PPE) is an integral step in the pyrolysis of PPE, which is a model compound for the ß-O-4 linkage in lignin. We investigated the influence of natural occurring substituents (hydroxy, methoxy) on the reaction rate by calculating relative rate constants using density functional theory in combination with transition state theory, including anharmonic correction for low-frequency modes. The phenyl-shift reaction proceeds through an oxaspiro[2.5]octadienyl radical intermediate and the overall rate constants were computed invoking the steady-state approximation (its validity was confirmed). Substituents on the phenethyl ring have only little influence on the rate constants. If a methoxy substituent is located in the para position of the phenyl ring adjacent to the ether oxygen, the energies of the intermediate and second transition state are lowered, but the overall rate constant is not significantly altered. This is a consequence of the dominating first transition from reactant to intermediate in the overall rate constant. In contrast, o- and di-o-methoxy substituents significantly accelerate the phenyl-migration rate compared to ß-PPE.

Characterization of biochars produced from cornstovers for soil amendment.

Through cation exchange capacity assay, nitrogen adsorption-desorption surface area measurements, scanning electron microscopic imaging, infrared spectra and elemental analyses, we characterized biochar materials produced from cornstover under two different pyrolysis conditions, fast pyrolysis at 450 °C and gasification at 700 °C. Our experimental results showed that the cation exchange capacity (CEC) of the fast-pyrolytic char is about twice as high as that of the gasification char as well as that of a standard soil sample. The CEC values correlate well with the increase in the ratios of the oxygen atoms to the carbon atoms (O:C ratios) in the biochar materials. The higher O:C ratio was consistent with the presence of more hydroxyl, carboxylate, and carbonyl groups in the fast pyrolysis char. These results show how control of biomass pyrolysis conditions can improve biochar properties for soil amendment and carbon sequestration. Since the CEC of the fast-pyrolytic cornstover char can be about double that of a standard soil sample, this type of biochar products would be suitable for improvement of soil properties such as CEC, and at the same time, can serve as a carbon sequestration agent.

Computational study of bond dissociation enthalpies for lignin model compounds. Substituent effects in phenethyl phenyl ethers.

Lignin is an abundant natural resource that is a potential source of valuable chemicals. Improved understanding of the pyrolysis of lignin occurs through the study of model compounds for which phenethyl phenyl ether (PhCH(2)CH(2)OPh, PPE) is the simplest example representing the dominant beta-O-4 ether linkage. The initial step in the thermal decomposition of PPE is the homolytic cleavage of the oxygen-carbon bond. The rate of this key step will depend on the bond dissociation enthalpy, which in turn will depend on the nature and location of relevant substituents. We used modern density functional methods to calculate the oxygen-carbon bond dissociation enthalpies for PPE and several oxygen-substituted derivatives. Since carbon-carbon bond cleavage in PPE could be a competitive initial reaction under high-temperature pyrolysis conditions, we also calculated substituent effects on these bond dissociation enthalpies. We found that the oxygen-carbon bond dissociation enthalpy is substantially lowered by oxygen substituents situated at the phenyl ring adjacent to the ether oxygen. On the other hand, the carbon-carbon bond dissociation enthalpy shows little variation with different substitution patterns on either phenyl ring.

Assessment of a computer-based treatment for older amblyopes: the Glasgow Pilot Study.

PURPOSE: There have been few viable alternatives to patching the better eye as a treatment of amblyopia for more than two centuries. The success of patching depends on compliance, which is problematic for up to 59% of children and their families. METHODS: This pilot study trialled the interactive binocular treatment (I-BiT) system as an alternative amblyopia treatment in 12 older amblyopes (6.1-11.4 years, median 8.2), who had not complied with or responded to occlusion. Virtual reality images were projected to each eye simultaneously via a headset during eight treatment sessions of 25-min duration. Outcome measures were changes in high- (HCVA) and low-contrast log MAR acuity (LCVA) at 1 week, 4 weeks and a final follow-up (3-18 months) after the final treatment. RESULTS: Sustained improvements in HCVA were observed in seven children (58%) and in LCVA in eight children (67%), including two for whom amblyopia was eliminated. Five children had visual acuities equivalent to 6/12 or better at least 6 months after stopping treatment, compared with one child prior to treatment. Significant improvements in HCVA occurred up to the fourth treatment; in LCVA to the seventh treatment. CONCLUSION: Sustained improvements in visual acuity were observed for 58% of this small group of children using the I-BiT system, despite prior failure with conventional treatment. This offers hope for a potential time-saving alternative to patching, in which compliance can easily be monitored, but the results need to be validated by means of a randomised controlled trial.

Computational prediction of alpha/beta selectivities in the pyrolysis of oxygen-substituted phenethyl phenyl ethers.

Phenethyl phenyl ether (PPE; PhCH 2CH 2OPh) is the simplest model for the most common beta-O-4 linkage in lignin. Previously, we developed a computational scheme to calculate the alpha/beta product selectivity in the pyrolysis of PPE by systematically exploiting error cancellation in the computation of relative rate constants. The alpha/beta selectivity is defined as the selectivity between the competitive hydrogen abstraction reaction paths on the alpha- and beta-carbons of PPE. We use density functional theory and employ transition state theory where we include diagonal anharmonic correction in the vibrational partition functions for low frequency modes for which a semiclassical expression is used. In this work we investigate the effect of oxygen substituents (hydroxy, methoxy) in the para position on the phenethyl ring of PPE on the alpha/beta selectivities. The total alpha/beta selectivity increases when substituents are introduced and is larger for the methoxy than the hydroxy substituent. The strongest effect of the substituents is observed for the alpha-pathway of the hydrogen abstraction by the phenoxyl chain carrying radical for which the rate increases. For the beta pathway and the abstraction by the R-benzyl radical (R = OH,OCH 3) the rate decreases with the introduction of the substituents. These findings are compared with results from recent experimental studies.

Kinetic analysis of the pyrolysis of phenethyl phenyl ether: computational prediction of alpha/beta-selectivities.

We calculated an overall alpha/beta-selectivity for the pyrolysis of phenethyl phenyl ether as a composite of the alpha/beta-selectivities in the hydrogen abstraction reactions by the phenoxyl and by the benzyl radical that is in excellent agreement with experiment. The difference between the individual selectivities for these radicals is explained by analyzing the electronic structure of the transition states. Spin delocalization of the single electron favors the alpha-pathways. An opposing effect occurs for polarized transition states, such as the transition states for the hydrogen abstraction by the electrophilic phenoxyl radical, where the adjacent ether oxygen in phenethyl phenyl ether stabilizes the beta-transition states. These results indicate that theory will be able to provide excellent predictions of alpha/beta-product selectivities for more complicated lignin model compounds bearing multiple substituents. We have developed a scheme to predict alpha/beta-product selectivities in the pyrolysis of model compounds for the beta-ether linkage in lignin. The approach is based on computation of the relative rate constant, which profits from error cancellation in the individual rate constants. The Arrhenius prefactors depend strongly on the description of the low-frequency modes for which anharmonic contributions are important. We use density functional theory in combination with transition-state theory in this analysis. Diagonal anharmonic effects for individual low-frequency modes are included by employing a second-order Wigner-Kirkwood expansion in a semiclassical expression for the vibrational partition function. The composite alpha/beta-product selectivity is obtained by applying quasi-steady-state kinetic analysis for the intermediate radicals.

Confinement effects on product selectivity in the pyrolysis of phenethyl phenyl ether in mesoporous silica.

Pyrolysis of phenethyl phenyl ether confined in mesoporous silicas by covalent grafting results in significantly increased product selectivity compared with fluid phases.