If one author publish same paper of same compound with same application or make only slight changes and publish papers in different journal. That kind of papers are under plagiarism or not.
Yes, if someone publishes his/her research twice, that's not ethical and it could also be in conflict with the copyright agreement signed with the first publisher.
If the "slight changes" are in the compound structure in the second publication, then perhaps it's not so clear that it's self-plagiarism, but in my opinion it wouldn't be ethical either, but if those "small changes" are in the manuscript, then that's clearly wrong.
It is very important to note that this kind of question can be best responded to with full information of the specific context of the two papers referred to. If for example, both papers belong to the same author and they are submitted to two different journal publishers; then it is more of an ethical problem and less of plagiarism. First of all, plagiarism, which is an infringement or a violation of another author’s copyright, is essentially a deliberate effort to knowingly copy and paste another author’s work without acknowledging it while masquerading as the author of the same. Honesty is still the best principle for all the authors.
In addition, it involves copying part of the article and using it in a similar context in such a way that a reader, who is exposed to both works, will be confused about the true identity of the original author of the work. In this case, there is no plea of ignorance on the part of the one who has plagiarized or copied from the original author. Besides, we usually assure the journal editors that our articles followed the required intellectual property rights (IPR).
Furthermore, there is a crucial a need to clarify or establish the context of this question. This because, part of the reason is that, in communication, the same words or phrases can be used with completely different meanings, depending on the contexts. On one hand, I find it very difficult to appreciate why anyone would like to practice plagiarism simply because, by merely citing or quoting another author’s work, the quality or credibility of the work of the citing or second author is automatically enhanced by it (the citation).
On the other hand, the quality of any literary work is diminished by its lack of authority which is usually derived from any citations of the others’ works. Let me also clarify that, even if one is copying from his or her own work, it is required to cite one’s own work as well. The more we choose to uphold high ethical code of practice and self-regulation as authors, the more plagiarism and its legal or perilous moral implications are abolished from the world.
In conclusion, it is possible two people working on the same project to write different papers based on the findings or results without copying from each other. This is the kind of thing that when we teach our children the same syllabus and when they do their assignments or do their exams, they write the same points but from their individual perspectives. Plagiarism is a legal matter based on IPR laws while submitting the same work to two different publishers, is an ethical issue.