无法将xpathApply应用于R中类“list”的对象

I am trying to query PubMed with a list of PMIDs and trying to get some information out for each article such as the publication type. I would like to ultimately create a data frame consisting of 2 columns: one listing the PMIDs and the other listing the publication type. Some publications have more than one publication type so a PMID should be replicated for each publication type. For example:I am trying to query PubMed with a list of PMID