Publications by authors named "Helen Long"

20 Publications

  • Page 1 of 1

Functional Communication Abilities in Youth With Cerebral Palsy: Association With Impairment Profiles and School-Based Therapy Goals.

Lang Speech Hear Serv Sch 2022 Jan 12;53(1):88-103. Epub 2021 Nov 12.

Waisman Center, University of Wisconsin-Madison.

Purpose: The aim of this study was to examine the relationship between functional communication skills, underlying speech, language, and cognitive impairments and school-based speech pathology services in students with cerebral palsy (CP).

Method: Thirty-five participants with CP who had Individualized Education Programs (IEPs) were classified according to the Communication Function Classification System (CFCS). Participants completed laboratory assessments of speech, receptive language, executive functioning, and nonverbal cognition. Each participant's speech and language IEP goals were coded into treatment units and then categorized into seven, mutually exclusive target goal areas. Nonparametric analyses were employed to examine differences among CFCS groups in the number of deficit areas and the number of goal areas from the IEP. Descriptive analyses were used to evaluate the extent to which deficit and goal areas in the IEP co-occurred by CFCS level.

Results: Those in more involved CFCS levels demonstrated more severe speech, receptive language, and cognitive impairments. However, there were no significant differences in the number of deficit areas across CFCS groups. Regardless of CFCS level, there were no differences in the number of treatment goals specified in the IEP. Literacy was the only goal area addressed across all CFCS levels. Those in the most involved CFCS levels had augmentative and alternative communication (AAC) goals, but those with more moderate restrictions in functional communication who also had markedly reduced speech intelligibility did not typically have speech or AAC goals.

Interpretation: Individuals with CP across CFCS levels demonstrate variability in underlying deficit profiles, suggesting that measures of both functional communication and of underlying deficits are necessary to provide a complete picture of communication needs. Literacy goals were common across all CFCS levels, but AAC goals were limited to the most severely involved individuals, suggesting that continuing education may be necessary to support speech-language pathologists in developing treatments involving the integration of AAC and speech to foster functional communication at school.

Supplemental Material: https://doi.org/10.23641/asha.16968073.
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http://dx.doi.org/10.1044/2021_LSHSS-21-00064DOI Listing
January 2022

Early Vocal Development in Tuberous Sclerosis Complex.

Pediatr Neurol 2021 12 10;125:48-52. Epub 2021 Sep 10.

University of Memphis, School of Communication Sciences and Disorders, Institute for Intelligent Systems, Memphis, Tennessee; Konrad Lorenz Institute for Evolution and Cognition Research, Klosterneuburg, Austria.

Background: Our goal was to assess for the first time early vocalizations as precursors to speech in audio-video recordings of infants with tuberous sclerosis complex (TSC).

Methods: We randomly selected 40 infants with TSC from the TSC Autism Center of Excellence Research Network dataset. Using human observers, we analyzed 74 audio-video recordings within a flexible software-based coding environment. During the recordings, infants were engaged in developmental testing. We determined syllables per minute (volubility), the number of consonant-vowel combinations, such as 'ba' (canonical babbling), and the canonical babbling ratio (canonical syllables/total syllables) and compared the data with two groups of typically developing (TD) infants. One comparison group's data had come from a laboratory setting, while the other's had come from all-day Language Environment Analysis recordings at home.

Results: Compared with TD infants in laboratory and all-day Language Environment Analysis recordings, entry into the canonical babbling stage was delayed in the majority of infants with TSC, and the canonical babbling ratio was low (TD mean = 0.346, SE = 0.19; TSC mean = 0.117, SE = 0.023). Volubility level in infants with TSC was less than half that of TD infants (TD mean = 9.82, SE = 5.78; TSC mean = 3.99, SE = 2.16).

Conclusions: Entry into the canonical stage and other precursors of speech development were delayed in infants with TSC and may signal poor language and developmental outcomes. Future studies are planned to assess prediction of language and developmental outcomes using these measures in a larger sample and in more precisely comparable recording circumstances.
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http://dx.doi.org/10.1016/j.pediatrneurol.2021.08.009DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8557126PMC
December 2021

The origin of language and relative roles of voice and gesture in early communication development.

Infant Behav Dev 2021 11 7;65:101648. Epub 2021 Oct 7.

School of Communication Sciences and Disorders, University of Memphis, Memphis, TN, USA; Konrad Lorenz Institute for Evolution and Cognition Research, Klosterneuburg, Austria; Institute for Intelligent Systems, University of Memphis, Memphis, TN, USA.

Both vocalization and gesture are universal modes of communication and fundamental features of language development. The gestural origins theory proposes that language evolved out of early gestural use. However, evidence reported here suggests vocalization is much more prominent in early human communication than gesture is. To our knowledge no prior research has investigated the rates of emergence of both gesture and vocalization across the first year in human infants. We evaluated the rates of gestures and speech-like vocalizations (protophones) in 10 infants at 4, 7, and 11 months of age using parent-infant laboratory recordings. We found that infant protophones outnumbered gestures substantially at all three ages, ranging from >35 times more protophones than gestures at 3 months, to >2.5 times more protophones than gestures at 11 months. The results suggest vocalization, not gesture, is the predominant mode of communication in human infants in the first year.
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http://dx.doi.org/10.1016/j.infbeh.2021.101648DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8669728PMC
November 2021

Protophones, the precursors to speech, dominate the human infant vocal landscape.

Philos Trans R Soc Lond B Biol Sci 2021 10 6;376(1836):20200255. Epub 2021 Sep 6.

School of Communication Sciences and Disorders, University of Memphis, Memphis, TN, USA.

Human infant vocalization is viewed as a critical foundation for vocal learning and language. All apes share distress sounds (shrieks and cries) and laughter. Another vocal type, speech-like sounds, common in human infants, is rare but not absent in other apes. These three vocal types form a basis for especially informative cross-species comparisons. To make such comparisons possible we need empirical research documenting the frequency of occurrence of all three. The present work provides a comprehensive portrayal of these three vocal types in the human infant from longitudinal research in various circumstances of recording. Recently, the predominant vocalizations of the human infant have been shown to be speech-like sounds, or 'protophones', including both canonical and non-canonical babbling. The research shows that protophones outnumber cries by a factor of at least five based on data from random-sampling of all-day recordings across the first year. The present work expands on the prior reports, showing the protophones vastly outnumber both cry and laughter in both all-day and laboratory recordings in various circumstances. The data provide new evidence of the predominance of protophones in the infant vocal landscape and illuminate their role in human vocal learning and the origin of language. This article is part of the theme issue 'Vocal learning in animals and humans'.
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http://dx.doi.org/10.1098/rstb.2020.0255DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8419580PMC
October 2021

Linking the obesity rs1421085 variant circuitry to cellular, metabolic, and organismal phenotypes in vivo.

Sci Adv 2021 Jul 21;7(30). Epub 2021 Jul 21.

Mammalian Genetics Unit, MRC Harwell Institute, Oxfordshire OX11 0RD, UK.

Variants in FTO have the strongest association with obesity; however, it is still unclear how those noncoding variants mechanistically affect whole-body physiology. We engineered a deletion of the rs1421085 conserved cis-regulatory module (CRM) in mice and confirmed in vivo that the CRM modulates and gene expression and mitochondrial function in adipocytes. The CRM affects molecular and cellular phenotypes in an adipose depot-dependent manner and affects organismal phenotypes that are relevant for obesity, including decreased high-fat diet-induced weight gain, decreased whole-body fat mass, and decreased skin fat thickness. Last, we connected the CRM to a genetically determined effect on steroid patterns in males that was dependent on nutritional challenge and conserved across mice and humans. Together, our data establish cross-species conservation of the rs1421085 regulatory circuitry at the molecular, cellular, metabolic, and organismal level, revealing previously unknown contextual dependence of the variant's action.
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http://dx.doi.org/10.1126/sciadv.abg0108DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8294759PMC
July 2021

Endogenous endophthalmitis following computerised tomography colonography.

N Z Med J 2021 06 25;134(1537):98-99. Epub 2021 Jun 25.

Consultant Ophthalmologist, Ophthalmology Department, Capital and Coast District Health Board, Wellington.

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June 2021

Panuveitis in Sweet syndrome.

Indian J Ophthalmol 2020 Sep;68(9):2004-2006

Eye Department, Capital and Coast District Health Board, Wellington, New Zealand.

Ocular manifestations of Sweet syndrome, or acute febrile neutrophilic dermatosis, are usually limited to the anterior segment. We report the case of a patient with bilateral panuveitis and retinal vasculitis associated with Sweet syndrome. A 45-year-old Asian female with an undiagnosed febrile illness with rash presented with bilateral panuveitis with haemorrhagic occlusive retinal vasculitis. Skin biopsy confirmed Sweet Syndrome. Intraocular inflammation resolved with a combination of topical and systemic corticosteroids as well as intravenous cyclophosphamide, with resulting permanent severe right visual impairment. Although an uncommon condition, Sweet syndrome should be considered in any febrile patient with skin lesions and uveitis.
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http://dx.doi.org/10.4103/ijo.IJO_1667_19DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7690461PMC
September 2020

Social and endogenous infant vocalizations.

PLoS One 2020 5;15(8):e0224956. Epub 2020 Aug 5.

Origins of Language Laboratory, School of Communication Sciences and Disorders, University of Memphis, Memphis, Tennessee, United States of America.

Research on infant vocal development has provided notable insights into vocal interaction with caregivers, elucidating growth in foundations for language through parental elicitation and reaction to vocalizations. A role for infant vocalizations produced endogenously, potentially providing raw material for interaction and a basis for growth in the vocal capacity itself, has received less attention. We report that in laboratory recordings of infants and their parents, the bulk of infant speech-like vocalizations, or "protophones", were directed toward no one and instead appeared to be generated endogenously, mostly in exploration of vocal abilities. The tendency to predominantly produce protophones without directing them to others occurred both during periods when parents were instructed to interact with their infants and during periods when parents were occupied with an interviewer, with the infants in the room. The results emphasize the infant as an agent in vocal learning, even when not interacting socially and suggest an enhanced perspective on foundations for vocal language.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0224956PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7406057PMC
September 2020

Infant boys are more vocal than infant girls.

Curr Biol 2020 05;30(10):R426-R427

Marcus Autism Center, Children's Healthcare of Atlanta, Atlanta, GA 30329, USA; Division of Autism and Related Disabilities, Department of Pediatrics, Emory University, School of Medicine, Atlanta, GA 30329, USA; Center for Translational Social Neuroscience, Emory University, Atlanta, GA 30329, USA.

Although it is generally assumed females have a language advantage over males, Oller et al., studying all-day recordings of 100 infants, found that boys in the first year of life produced more speech-like vocalizations than girls and that the effect size was more than four times larger than the commonly reported female language advantage.
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http://dx.doi.org/10.1016/j.cub.2020.03.049DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8204662PMC
May 2020

Preterm and full term infant vocalization and the origin of language.

Sci Rep 2019 10 14;9(1):14734. Epub 2019 Oct 14.

Alpert Medical School of Brown University, Women and Infants Hospital, Providence, RI, USA.

How did vocal language originate? Before trying to determine how referential vocabulary or syntax may have arisen, it is critical to explain how ancient hominins began to produce vocalization flexibly, without binding to emotions or functions. A crucial factor in the vocal communicative split of hominins from the ape background may thus have been copious, functionally flexible vocalization, starting in infancy and continuing throughout life, long before there were more advanced linguistic features such as referential vocabulary. 2-3 month-old modern human infants produce "protophones", including at least three types of functionally flexible non-cry precursors to speech rarely reported in other ape infants. But how early in life do protophones actually appear? We report that the most common protophone types emerge abundantly as early as vocalization can be observed in infancy, in preterm infants still in neonatal intensive care. Contrary to the expectation that cries are the predominant vocalizations of infancy, our all-day recordings showed that protophones occurred far more frequently than cries in both preterm and full-term infants. Protophones were not limited to interactive circumstances, but also occurred at high rates when infants were alone, indicating an endogenous inclination to vocalize exploratorily, perhaps the most fundamental capacity underlying vocal language.
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http://dx.doi.org/10.1038/s41598-019-51352-0DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6791864PMC
October 2019

Patterns of Variation and Chemosystematic Significance of Phenolic Compounds in the Genus (Fabaceae, Podalyrieae).

Molecules 2019 Jun 26;24(13). Epub 2019 Jun 26.

Department of Botany and Plant Biotechnology, University of Johannesburg, P.O. Box 524, Auckland Park, Johannesburg 2006, South Africa.

As a contribution towards a better understanding of phenolic variation in the genus (honeybush tea), a collection of 82 samples from 15 of the 23 known species was analysed using liquid-chromatography-high resolution mass spectrometry (UPLC-HRMS) in electrospray ionization (ESI) negative mode. Mangiferin and isomangiferin were found to be the main compounds detected in most samples, with the exception of and where none of these compounds were detected. These xanthones were found to be absent from the seeds and also illustrated consistent differences between species and provenances. Results for contemporary samples agreed closely with those based on analysis of a collection of ca. 30-year-old samples. The use of multivariate tools allowed for graphical visualizations of the patterns of variation as well as the levels of the main phenolic compounds. Exclusion of mangiferin and citric acid from the data was found to give better visual separation between species. The use of UPLC-HRMS generated a large dataset that allowed for comparisons between species, provenances and plant parts (leaves, pods, flowers and seeds). Phenetic analyses resulted in groupings of samples that were partly congruent with species but not with morphological groupings within the genus. Although different provenances of the same species were sometimes found to be very variable, Principle Component Analysis (PCA) indicated that a combination of compounds have some (albeit limited) potential as diagnostic characters at species level. 74 Phenolic compounds are presented, many of which were identified for the first time in species, with nine of these being responsible for the separation between samples in the PCAs.
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http://dx.doi.org/10.3390/molecules24132352DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6651507PMC
June 2019

Reliability of Listener Judgments of Infant Vocal Imitation.

Front Psychol 2019 11;10:1340. Epub 2019 Jun 11.

Department of Mathematical Sciences HL, University of Memphis, Memphis, TN, United States.

There are many theories surrounding infant imitation; however, there is no research to our knowledge evaluating the reliability of listener perception of vocal imitation in prelinguistic infants. This paper evaluates intra- and inter-rater judgments on the degree of "imitativeness" in utterances of infants below 12 months of age. 18 listeners were presented audio segments selected from naturalistic recordings to represent in each case a parent vocal model followed by an infant utterance ranging from low to high degrees of imitativeness. The naturalistic data suggested vocal imitation occurred rarely across the first year, but strong intra- and inter-rater correlations were found for judgments of imitativeness. Our results suggest salience of the infant's vocal imitation despite its rare occurrence as well as active perception by listeners of the imitative signal. We discuss infant vocal imitation as a potential signal of well-being as perceived by caregivers.
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http://dx.doi.org/10.3389/fpsyg.2019.01340DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6579846PMC
June 2019

Analysis of Phenolic Compounds in Rooibos Tea (Aspalathus linearis) with a Comparison of Flavonoid-Based Compounds in Natural Populations of Plants from Different Regions.

J Agric Food Chem 2017 Nov 13;65(47):10270-10281. Epub 2017 Nov 13.

Department of Botany and Plant Biotechnology, University of Johannesburg , P.O. Box 524, Auckland Park, Johannesburg, 2006 South Africa.

Tea samples from 17 populations of "wild tea" ecotypes Aspalathus linearis (rooibos tea) and 2 populations of Aspalathus pendula were analyzed. Recent advances in column technology together with high-resolution mass spectrometry were applied to improve resolution, facilitating the identification of several new compounds as well as grouping of the wild tea ecotypes according to their chemical composition. The collisional cross-section data obtained from ion mobility-mass spectrometry is reported for the flavonoids in rooibos for the first time. Enzyme pathways for the synthesis of the unique flavonoids found in rooibos tea are also proposed. A. linearis and A. pendula produce similar combinations of main phenolic compounds, with no diagnostically different discontinuities between populations or species. Northern resprouters (Gifberg and Nieuwoudtville) contain higher phenylpropenoic acid glucoside levels while teas from Wupperthal and surrounding areas were found to contain unique dihydrochalcones (phloridzin and a sieboldin analog), which are reported here for the first time.
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http://dx.doi.org/10.1021/acs.jafc.7b03942DOI Listing
November 2017

Informal caregiving for elders in Sweden: an analysis of current policy developments.

J Aging Soc Policy 2011 Oct;23(4):335-53

Research Coordinator, The National Board of Health and Welfare, Stockholm, Sweden.

In Sweden, care of elderly people is a public responsibility. There are comprehensive public policies and programs providing health care, social services, pensions, and other forms of social insurance. Even so, families are still the major providers of care for older people. In the 1990s, the family was "rediscovered" regarding eldercare in Sweden. New policies and legislative changes were promoted to support family caregivers. The development of services and support for caregivers at the municipal level has been stimulated through the use of national grants. As a result, family caregivers have received more recognition and are now more visible. However, the "Swedish model" of publicly financed services and universal care has difficulty addressing caregivers. Reductions in institutional care and cutbacks in public services have had negative repercussions for caregivers and may explain why research shows that family caregiving is expanding. At the same time, a growing "caregivers movement" is lobbying local and national governments to provide more easily accessible, flexible, and tailored support. In 2009, the Swedish Parliament passed a new law that states: "Municipalities are obliged to offer support to persons caring for people with chronic illnesses, elderly people, or people with functional disabilities." The question is whether the new legislation represents a paradigm shift from a welfare system focused on the individual to a more family-oriented system. If so, what are the driving forces, motives, and consequences of this development for the different stakeholders? This will be the starting point for a policy analysis of current developments in family caregiving of elderly people in Sweden.
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http://dx.doi.org/10.1080/08959420.2011.605630DOI Listing
October 2011

Self-management of health-behaviors among older and younger workers with chronic illness.

Patient Educ Couns 2009 Oct 8;77(1):109-15. Epub 2009 Apr 8.

Department of Human Sciences, Brockington Building, Loughborough University, Leicestershire, UK.

Objective: To examine the self-management of health behaviors carried out by older (aged 50-69 years) and younger workers (aged 20-49 years) with a chronic illness.

Methods: Questionnaire data was collected from 759 employees with a diagnosed chronic illness. Four categories of self-managing health behaviors were examined: using prescribed medication, monitoring and responding to symptoms, managing an appropriate diet and exercising.

Results: The majority of participants (56-97%) reported being advised to carry out health behaviors at home and at work. Controlling for confounding factors, medication use was associated with younger and older workers. Managing an appropriate diet was associated with younger workers with asthma, musculoskeletal pain or diabetes. Exercising was associated with younger workers with asthma and with older workers with heart disease, arthritis and rheumatism or diabetes.

Conclusions: The findings indicate that there are differences in diet and exercise activities among younger and older workers.

Practice Implications: To increase self-management in health behaviors at work, improved communication and understanding between the different health professions and the patient/employee is required so that different tailored approaches can be effectively targeted both by age and within the context of the working environment, to those managing asthma, heart disease, diabetes and arthritis and rheumatism.
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http://dx.doi.org/10.1016/j.pec.2009.02.014DOI Listing
October 2009

Five-year experience of corneal scrapes at Wellington Eye Department, New Zealand.

N Z Med J 2007 Aug 24;120(1260):U2682. Epub 2007 Aug 24.

Ophthalmology Unit, Department of Surgery and Anaesthesia, Wellington School of Medicine, University of Otago, Wellington.

Aim: To determine the causative organisms of bacterial keratitis in Wellington and to identify the antibiotic sensitivities of each bacterium isolated. These results will then be compared with certain patient characteristics and clinical outcomes.

Methods: Corneal scrapes collected between 2001 and 2005 were retrospectively analysed and collated on a database. Corneal scrapes were collected by an ophthalmologist and processed by trained microbiological staff.

Results: 34 scrapes were collected; there was a positive Gram-stain in 38% of cases. A positive culture was obtained in 85% of scrapes. The commonest Gram-negative organism was Moraxella spp. (12.5%). The commonest Gram-positive organism was coagulase-negative Staphylococci (25%). The Gram-negative and Gram-positive bacteria were all sensitive to chloramphenicol. Ocular surface disease was the most common risk factor.

Conclusion: Our study shows similarities and differences with other ophthalmology centres around the World, this emphasises the recognised regional variation of bacterial keratitis. The two most important points to be taken from our results are: an accurate database for recording corneal scrape details and a protocol for testing antibiotic sensitivities and resistance needs to be established in Wellington; and a future study needs to be carried out on the next five years of corneal scrapes.
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August 2007

Work factors related to psychological and health-related distress among employees with chronic illnesses.

J Occup Rehabil 2007 Jun 28;17(2):259-77. Epub 2007 Feb 28.

Department of Human Sciences, Brockington Building, Loughborough University, Loughborough, Leicestershire, LE11 3TU, UK.

Objective: This study examined specific psychosocial factors associated with psychological and health-related distress amongst employees reporting different chronic illnesses.

Methods: The sample consisted of 1029 employees managing either musculoskeletal pain (n=324), arthritis and rheumatism (n=192), asthma (n=174), depression and anxiety (n=152), heart disease (n=96) or diabetes (n=91). Information on psychological distress, work limitations, illness management, disclosure, absence, presenteeism, support and demographic factors were obtained through self-administered questionnaires.

Results: Both low psychological well-being and high health-related distress were associated with an increase in work limitations (beta=0.20, SE=.03; and beta=0.19, SE=.01, respectively), poorer management of illness symptoms at work (beta=-0.17, SE=.12; and beta=-0.13, SE=.02), high presentieesm (beta=0.19, SE=.25; and beta=0.14, SE=.05) and low workplace support (beta=-0.05, SE=.22; and beta=-0.12, SE=.05). Health-related distress was additionally associated with disclosure of illness at work (beta=0.18, SE=.08) and long-term sickness absence (beta=0.10, SE=.06).

Conclusions: To enable individuals to effectively manage both their illness and their work without serious repercussions, it is important for both healthcare professionals and employers alike, to improve the well-being of workers with chronic illness by supporting and facilitating their efforts to over-come health-related limitations at work.
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http://dx.doi.org/10.1007/s10926-007-9074-3DOI Listing
June 2007

Presumed CMV associated necrotizing retinopathy in a non-HIV immunocompromised host.

Clin Exp Ophthalmol 2005 Jun;33(3):330-2

Herpetic necrotizing retinitis is caused by the herpesvirus group of viruses, including cytomegalovirus (CMV). Reports defining causative viruses have shown CMV to be a rare cause of acute retinal necrosis. In fact, there is only one report in the literature of CMV associated with progressive outer retinal necrosis, which is most commonly associated with Varicella zoster virus. A case is presented of CMV-related necrotizing retinopathy concurrent with subtle signs of herpetic retinitis in an immunocompromised patient, highlighting the spectrum of response of herpetic retinopathies.
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http://dx.doi.org/10.1111/j.1442-9071.2005.00996.xDOI Listing
June 2005

Investigations by (13)C NMR spectroscopy of ethene-initiated catalytic CO hydrogenation.

J Am Chem Soc 2002 Sep;124(35):10456-72

Department of Chemistry, The University of Sheffield, Sheffield S3 7HF, UK.

13C NMR spectroscopy shows that the n-alkene and n-alkane products from the catalytic hydrogenation of CO in the presence of (13)C(2)H(4) probes over Ru/150 degrees C, Co/180 degrees C, Fe/220 degrees C, or Rh/190 degrees C (1 atm, CO:H(2) 1:1, "mild conditions") contain terminal (13)CH(3)(13)CH(2)- units. This is consistent with their formation by a regiospecific polymerization of C(1) species derived from CO and initiated by (13)C(2)H(4). Although the activities toward individual products differed somewhat, similar distributions and similar product labeling patterns were obtained over all the four catalysts. 1-Butene and the higher 1-n-alkenes from all the catalysts were largely (13)CH(3)(13)CH(2)(CH(2))(n)()CH=CH(2) (n = 0-3), propene formed over Ru or Co was (13)CH(3)(13)CH=CH(2), while both (13)CH(3)(13)CH=CH(2) and (13)CH(2)=(13)CHCH(3) were formed over Fe or Rh. Comparison of the conclusions from these probe experiments with those from isotope transient experiments by other workers indicates that the ethene initiator does not significantly modify the course of the CO hydrogenation. The reaction products are largely kinetically determined, and the primary products are mainly linear 1-n-alkenes, while the n-alkanes and 2-n-alkenes largely arise via secondary processes. Since the distribution of products and the labeling in them is so similar, it is concluded that one basic primary mechanism applies over all the four metals. Several different reaction paths involving a polymerization of surface methylene, [CH(2(ad))], have been proposed. Although the predictions based on several of these mechanisms agree with many of the results, the alkenyl + [CH(2(ad))] mechanism, initiated by a surface vinyl [CH(2)=CH((ad))], most easily accommodates the experimental evidence. An alternative path involving sequential addition of surface methylidyne and hydride either to a growing alkylidene chain (alkylidene + [CH(ad) + H(ad)]) or to an alkyl chain (alkyl + [CH((ad)) + H(ad)]) has recently been proposed by van Santen and Ciobica. The [CH(2(ad))] mechanism offers an easier explanation for the formation of the various alkenes, the distribution of products, and of the initiation, while the [CH(ad) + H(ad)] mechanism can explain any n-alkanes formed as primary products and not derived from alkenes. At higher reaction temperatures over Ru and Co, considerable (13)C(1) incorporation (from natural abundance in the CO and from cleavage of the (13)C(2)H(4) probe) was found in all the hydrocarbons. Thus, at higher temperatures (13)C(1(ad)) in addition to (13)C(2(ad)) species participate in both chain growth and initiation. In summary, adsorbed CO is transformed very easily into surface C(1(ad)), probably [CH(2(ad))] in equilibrium with [CH((ad))+H(ad)], which act as the propagating species.
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http://dx.doi.org/10.1021/ja026280vDOI Listing
September 2002
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